It is the same reaction as when you get attacked during a wild pet battle.
Everything just stops, and you are instantly pulled out of battle.
Perhaps someone inspecting me? Getting thrown out of battles until the other guy stops inspecting you, or just getting thrown out when someone clicks on the "inspect"-button?
I remember those bugs way back (and not so way back) where everyone in a zone sitting and eating/drinking was forced to stand up whenever something happened in that zone, such as a quest completion or whatever. Something like that happened in darkmoon as well (I might remember incorrectly). Perhaps it's something concerning this bug, where those thought-to-be-resolved bugs have arisen as bugs again after the pet battle system was implemented. But as it's only in darkmoon / master pet trainers as far as I understand from your reports, it's too inconsistent for this conclusion.
Strange bug. First time I experienced it though.