Class change mid game.

It seems that there are multiple triggers to randomly changing class mid game. One of the one I know from multiple experience is linked to drag and reviving a teammate. I think there is a class change linked to that action, where using q or e moves the selection to a different class. 

There are other triggers too, I just done know which. I know because I was using medic last night, and find out I changed class to engineer when trying to diffibulate a teammate, and ended up blowing myself up by shooting an RPG on the floor. Perhaps its the act of res-ing a teammate that triggers it?