It appears that they have some part of Clockwork Rook's code left in them. If 2 Hippos are walking very close to each other (while chasing me) they can apparently damage each other as if they were a charging Rooks. It even made a sound that Rook makes when it hits something.
Or maybe preaparing to jump caused it. Whatever- if 2 Hippos are close to each other and are aggroed to something they can damage each other.