Just throwing this out there.
The tank probably wouldn't quit if the DPS weren't woefully undergeared.
Considering they indicate that the tank quits before even entering, I doubt that is the case.
It probably has more to do with the fact that they misqueued as a tank (GF will sometimes reselect the Tank option) and they relized it when they got the group interface and saw they had the shield icon. I'm keyed to the issue and normally pick up on the "a group was found" screen, but you can sometimes miss it.