

I think he must have dropped at a very specific moment 
I know how this happens.
You know when game finds you a match and you are on loading screen. When the loading screen is about to come up and screen goes a bit black you can see who is joining the match, which means before you officially join the match/the loading screen starts you can see who your opponents are. And some players can exit the game by discontenting or alt+f4 the game, I guess. It happened to me to a few times, which makes me think most possibly in such scenario when a player has joined the game and not exit the game and since the game has not been cancelled (which it should imo, for it being unfair to the remaining player/s in the game), then system replaced the player with CPU easy. I'm almost sure that's the reason. people can do that when the think they are against stronger players they might know and they don't wanna be in such a match up