Title says it. Tank merc, J5, disengages from combat at random. No coward emote. It just stops fighting, walks over to player and sits there. Fortunately it happened at the same time pet agro was changed so we switched to using mage pet as MT and then last patch. We'll. Other threads cover that.
There's nothing fortunate about it. Don't mistake coincidence for causality. I'm a warrior, and I've noticed whenever I use my aggro pet, my melee dps merc will disengage too. It's not random, at least for me.
My merc, in level 85 zones, has been acting as if he has a Focus command on him. He ignores the mob that's beating on me until the first one is dead. I bugged it back in late Nov.