There are other sites where I base my avator on a particular athlete... video game... band....
Only on Sherdog do you really get quite as much shit for it though, lol.
Option 1 is respect : The person swithcing the Avatar was made af an and wants to show support
Option 2 is relevant avatar / current times. I've changed plenty from running backs to QB's to boxers, to GTA5, to something funny or political.
Option 3 is frontrunners to bandwagon.
Option 4 is bandwagon hoppers hoping to make people think they always had that avatar and knew fighter X was going to win, and post in super annoying fashion about how right they were.