Usually I have noticed this when the thrall is attacked and hit at specific times. Its more likely to happen when they are attacked before I am. But even then the event is usually quite uncommon. Its exacerbated by the fact that it can compound itself when they are continually being hit and only occasionally do they just attack without a weapon.

This gets worse if the server has gone more than 3 days without a restart (depending on many other factors as a fresh server can go ~4 days). A reboot might help here if its not already been done.