Having volunteered often over the years, there are many subjective factors that go into who gets selected but order of volunteering is generally bottom of the list. Generally I have found these subjective factors to be prioritized by
1) ease of reaccommodation to final destination (including whether you have checked bags)
2) whether you know the program and make it easy for the GA to get the flight out on time without having to explain things
3) how much the GA likes you
4) status with airline
5) where you are sitting on the plane (including how much you paid for your ticket, who will be taking your seat, whether they will need to mess around with having to upgrade someone and whether there is going to be space for your bag)
6) all else equal, who volunteered first