Could be any number of things. The heifer may just be a bully and is showing dominance. She could be getting close enough to calving that her horones are messing with her mind. Could be she's just playing (in her mind) with them. She could be food agressive and sees the calves as a challenge. Could be a combination of them all or none of those. That's part of what makes this business interesting, trying to figure out what is going on. If you have some older cows, especially a dominant one, you may want to put her with them for a while till the older cow beats the crap out of her. That sometimes will calm down the bullys, sometimes it makes it worse. I kind of hate to admit it, but I've been known to ship a heifer out that just won;t settle in to playing nice with the herd. Jeanne from Simmi Valley used to sue a term that I like. COD, Cull On Disposition. Disposition isn;t just behavior with the people working them, it also applies to fitting into the herd. That's my interpretation, everyone else may vary