Sounds like an old bug has resurfaced. I remember this being an issue some time last year, but I don’t remember if Funcom actually fixed it or anything because I got tired of waiting and left my hyena home and never got him out again.

I was never able to conclusively determine which conditions cause the hyena to stop attacking and remain stuck in a battle stance. Occasionally it happens to human followers too, but they usually recover their wits eventually and resume crushing their enemies and so on.