Enrage

Enrage is an ability that most monsters have to prevent them from being kept busy forever. It differs from Berserk in that it is unlikely to trigger during normal play.

If a monster that is in combat with you does not receive any damage at all for five minutes, it will enter an enrage state. During this state, every attack it does will land a critical strike. The enrage lasts until the monster resets, or is defeated.

This mechanic tries to prevent high level characters collecting low level mobs (which will virtually always miss), and then going away for a long time, depriving other players that need them of these monsters. Note that while this mechanic exists to prevent holding monsters for a long time, it does not make it okay to do so (especially when other players need those monsters and get frustrated by someone holding them). If this is a persistent problem, talk to whoever prevents you from killing monsters that way, or let us know by reporting them.