I have seen this a few times also and actually the tank in our group was one, he was doing nothing... So group vote dismissed him.
Other times i have noticed black mages beside me just stand there for a few minutes and then start to attack, or leave. Guessing they were trying to copy my rotations, lol

And also omg why are tanks so bad in ct, st and wod... Sorry small rant but i do more tanking as blm then most of them do