I've seen DPS withdraw, I've seen healers withdraw, I've seen tanks withdraw. You simply, through the luck of the draw, got four DPS that withdrew.

There are two reasons I can come up with for withdrawals. The first, is they are AFK. That's the withdrawal that you see them take forever to Accept, and they never do. The second is they are fishing for In Progress stuff. They don't know that you are trying to do The Navel, they figure they ar going to get something like Aurum Vale and would rather find something that has been mostly beaten to join.