What if they are a really advanced specie and see themselves as the mentors of the younger species, and think that only through conflict and war the species can evolve, so the weak die and only the stronger remain to breed into a new generation, more prepared to survive in the vast and dangerous galaxy? Thus, each hundred years they go out fighting everything in the galaxy, but without annihilating any planet completely. Also, they would have a strong warrior culture where dying in combat is the most honourable death, making the return of the warriors very unlikely, so their home world would have less defensive forces after the war season.
As a bonus point, they could be instigating conflicts between the other species behind the curtains while they are not in annihilation mode.
I think I recall some tv series having a somewhat similar thing as a plot...
