To exterminate us so that we dont have a chance to challenge them could be a motivation.
The universe is a big place. Looking around, it seems that the endless-exponential-expansion route hasn’t proven popular with anyone yet. There may just be enough to go around.
It doesn't need to be rational. All it takes is one sufficiently powerful species to decide "better safe than sorry" and lobbing suitable planet killers at any new upstarts they notice.
Now that you know it's there, don't you feel like expending the time, energy, and resources necessary to travel there so you can step on it?