I will describe the mechanic below. I look for ideas what race it is (name, type, why they are doing it, etc, etc).

There is a planet somewhere in the galaxy. It is a home of horrible alien creatures (probably not intelligent, but not necessarily). Most of the time they stay on their homeplanet doing nothing (or proparing or whatever, they just sit there). But, once every 100 years they start a new cycle (of mating maybe?) and spread across galaxy (proximity based, first to the planets nearby their homeplanets) and wrecak havock (taking over planets and decimating local population - but never eliminating them all). Then after several years their activity ends, they stop the attacks and go back to their homeplanet (and/or they slowly die out on all planets except their homeplanet). They are represented by colour green on the galactic map, their homeplanet has insane defences and can be conquered (thus eliminating the threat from the galaxy) only by attacking it just after the cycle ends (they become temporary weak at that time).

How to explain their strange behaviour? Is it some biological cycle for their race? Or maybe they are some sort of robots? Do they produce spaceships? Or do they have biological ships? And so on :)

Note: I'm not looking for super weird stuff, more for reasonable explanation and interesting stuff.

You dont spread through the galaxy without some level of intelligence, even having some biological tech. If they dont return to their planets after the exodus, it can be justified as some overpopulation control mechanism or perhaps some way to create "new swarms", like the bees. The old specimens leave home planet, leaving eggs or young ones.

If the specie is highly intelligent, it can be some sort of warrior tradition that evolved from simply conquering other local territories to conquering other planets.

How to explain their strange behaviour?

First thought: true lemming

A biological race, quite stupid and aggressive (killing mostly themself), but at times the population grows exponetilally and they need to swarm out.

Every 100 years a new leader is chosen through a game of hunting/conquest. Each prince/general/whatever is given control of one fleet, and the one who conquers the most territory in 10(?) years will be declared the new leader of the race. Honor is important to them, so they don't fight amongst each other. Their code requires them to keep at least 100,000,000(?) people alive on each planet to make sure the population can grow for the next games.

After the games, there will be 1 year(?) of celebration for the new leader where their guard is low. This is because normally they have conquered all the surrounding territory around this time, and they don't expect any actual threat to them.

The old specimens leave home planet, leaving eggs or young ones.

LOL, I like that one :)

Considering them not being smart enough to wonder why they stay at their planet for 100 years doing nothing, I'd consider a scenario these creatures being part of a lab experiment (yes sounds like Prometheus movie of Alien series ) which Empire comes there only to find a fully automated system by ancient race for ensuring that "this galaxy" will reset at regular intervals.

Going off of Ashaman's lemmings, a similar explanation for your aliens can be cicadas. Cicadas live and mature underground Some 'periodical' cicadas have huge (numerically) and wide-spread (geographically) broods that predictably emerge after a number of years pass (depending on the brood - different broods have different, but self-consistent, cycles). When they emerge, the entire brood emerges en-mass, and tens or hundreds of millions of cicadas swarm over the trees mating and laying eggs for a few weeks, shedding their skins, and then dying/going back underground.

They are predictable enough, we know what brood is going to emerge in what year. In terms of geographic spread, we're talking covering entire states. Wikipedia says some broods have 13 years underground, and others have 17 years underground before emerging.

In Kansas City, Missouri (USA), we just about every year have regular cicadas humming quietly in the background at certain times of the year. But in 2011, Brood XIX emerged - they hummed, all right, but it was anything but 'quiet'. Apart from the noise, they weren't any nuisance - you'd barely see them unless you are looking at the trees up-close, and they don't bite or sting or really do anything annoying. (Apparently this one 'brood' (Brood XIX) actually has four different species in it - I wasn't aware of that, but that may help your lore also).

I recall watching a documentary recently (I think it was a 'Planet Earth' episode) about the Taiga forest, and how one cyclic insect species comes out to mate, and is so plentiful they cover the trees, and the animals engorge themselves on the insects, and even then there are so many millions of insects that, once done laying their eggs, fall and cover the forest floor thickly enough to provide fertilizer for the trees.

Your alien species could go on a mating frenzy on their own home planet, lay their eggs on the home planet, and then they all leave the planet to go on a feeding frenzy before they die of natural causes leaving their corpses on foreign planets. I don't know why they'd need to go on a feeding frenzy after mating and right before dying, but this would conveniently get rid of them having to go out and then come back, and explains why they aren't spreading to those planets. Also, their mating season provides a natural "warning" to players before their "rampage" season begins.

It also would explain their really good defenses except after rampaging - normally, they are an intelligent species that sticks to their own planet, but have really good defenses to defend themselves from all the other species that hate them for their rampages. But when their rampages begin, they go crazy and all but abandon their home planet. Or perhaps they don't go crazy? Maybe, they are similar to salmon:

"Condition tends to deteriorate the longer the fish remain in fresh water, and they then deteriorate further after they spawn, when they are known as kelts. In all species of Pacific salmon, the mature individuals die within a few days or weeks of spawning, a trait known as semelparity."

So, the alien species mates on their own planet, and lay their eggs on their own planet, then, knowing they are going to die in several months (due to their own biology), leave only a small contingency on the planet as a rear-guard, and in their last few months of living, intentionally go on a rampage to weaken every other intelligent space-faring species, to set the other planets' development back far enough that the newly laid eggs have a chance to hatch and mature before they are able to defend themselves.

To pick up the threads and add some culture to an alien race:

The race is quite aggressive and are able to reproduce quite quickly. To strength their society it is always the ritual of the whole population to challenge the universe. In this case the whole population leave their homeplanet and bring devastation to the surrounding systems. After such a challenge, all survivors return to their home planet and restart the cycle (survival of the fittest).

How about the race has the ability to gather new DNA that makes them stronger and more adaptable etc. Once they have fully integrated the DNA gathered from last season, they go out to other planets and maybe return to the previous planet to see how that species changed.

In order to get their DNA they send in forces to collect people by the mass. The forces that they have set up on that planet do experiments with it on themselves. Some of them die trying to adapt to it, the others that live on are taken back to the home planet to integrate the new genetics with the rest of the planet.

This forced evolution makes them highly adaptable and smart. They are a force to be reckoned with.

As a plot device you could make it so that because they share some genes with X race that the main character is, he takes in some of their DNA using their technology. It gives him blah blah abilities.

They consider themselves the apex of the universe, and they are. Everything that we have mastered, they have too and more. They don't consider much of us. Maybe a few sympathizers.

Why not make them plant or even fungus based?

It takes them 100 years to grow to maturity at which point they pump spores into space, which settle on planets, spreading like a galactic cancer. They spread veins through the crust of the planet, and don't need ships as such as the spores themselves can resist interstellar space.

There could be some kind of explanation as to how they travel so fast, e.g. Having some kind of chemical makeup that mimics a star drives fuel source naturally and allows them to travel faster than light. It could also make them extremely volatile if attacked with conventional munitions.

Thoughts?