I have dropped groups for the dps doing this. I find it highly insulting. I'm either the tank all the way through it I'm not. I refuse to run with DPS who only let me fulfill my role when they deem it necessary. I will not be the meat shield you only pull out for the boss fights.
Then you were too slow in pulling, that's why it happened. Sure take your time before boss, but dont stand around before every pack, possibly asking "rdy?"