Some of the Tyranids' lore implies that they're actually fleeing from their original galaxy in order to escape something that's significantly more dangerous than they are. Whenever a group of 40k fans speculate about what that something could be you always get a mixture of serious answers and joke answers.

Kirby is frequently brought up as a joke answer for those unfamiliar to Kirby's lore and feats, but for those who know Kirby's lore it's a serious possibility.

THey know who the apex predator is.