[Pumpkin_RIP]

I have read that they usually do this to the "underdog" of a group. It isn't very common, and I'm not sure what you can really do to stop it.

I would check with someone else though, because I'm not completely sure that where I read that info is a reliable source. I just thought I would put that idea out there as a likely possibilty.

EDIT: since Pigsforlife says they do it from bordeum or jealously, maybe you could give them new toys to play with and spend more time with them? Just a suggestion.