hmm..
we didn't have a loot priority from MC to SSC/TK but started with is at BT und HS.
there's always the possibility to lose geared up people to other raids / server, no matter if they had loot priority or not. It doesn't really matter if the tank has 5/5 tier 6 oder just 4/5 an kaz-shield instead of the bullwark.
When I think back I have to say that most of the people that left our raid was DDs or healers.
Our tanks (including me) are raiding with us since the first ragnaros pulls or razorgore wipes. We lost exactly one (warri) tank during this time, and for that we have to blame RL
The other raids' situations are very similar to ours.
A "real" maintank or tank feels a kind of responsibility to the raid and the people, I think.
If 3 of your tanks left your raid, they would have done it even without loot priority, for sure. there must be other reasons than just progress so it was just a matter of time and you just had bad luck in this case.
I'm sorry if my english is kind of bad right now, but I'm from germany and it's deep night right now for me