The way i understand it is its a glitch with the mobs AI.
For instance if its targeting a pet or another play who dies mid-cast it will basically "Auto-target" the next person and if its mid-spell it will glitch and cause the cure spell to cast on you.
Also sometimes it just reads cure as an Offensive spell (Thanks to undead), and casts it on a player.
I've had Shadowhand cast Cure 1 on me in campaign, despite the fact... shes undead... and doesn't possess cure spells, that was a fun screenshot x.x
One of the trance belt/genie gages NMs casted cure on me right before it was going to kill me, the rdm one. I was at like 110ish health and it cured me to almost 600. If it ain't broken don't fix it!