I've found that if the lobby is full, or even if the teams just stay balanced, the game will swap the defensive and offensive teams each new session. In the event that a bunch of people leave one team, creating an unbalanced lobby the game appears to "start over", shuffle both teams together and then randomly reassign each player until the teams are even again. Because you have a 50/50 chance of getting assigned to the same team you were just on it's not unusual for this to happen even several times in a row because you can flip a coin and keep getting Heads or Tails repeatedly.
I've noticed the same and wondered whether it's a thing or if it is just as @realitysquared suggested. I find that you can usually swap teams if you do it quickly when the character selection screen appears. A few players often drop out right when the game is about to start.
