Not only aggression, but hallucination, in other cases. I fully suspect that to be the work of the fog, which means that all these recent troubles stem from nature itself...

[An image flashes by of an Orthworm defending a Tailow's medicine, thinking it's her child. The medicine in question looks like a worm.

Then there's a Persian, fighting off Ren and Firepaw, as they try to rescue Purrloin.]