You had 10 players dressed. If you are up by over 20 points in the 4th quarter, the game engine will try to get every starter out of the game, regardless of how you have filled in the depth chart. Confusing, I know, but that's how it seems to work.
If you had only dressed 8 players, there would have been no one available to sub in for A and B, so they would have stayed in the game and played all 48 minutes (unless they fouled out or were injured).