If you're using the original Kruthik fluff, they're already a weaponized colony based species who are apt at destroying everything in sight. Their very presence is a reason to get rid of them.

If this is the case, I'd go with the classic 'we dug too deep and too greedily' sort of thing, where an expansion/mining operation/cave in/explosion awoke a den of Kruthik (who breed like mad) and exposed the area to them.