There's sort of a silly argument as to how you could get mate. If a computer is playing black, it reads the board as greatly to its advantage. So when it's comi

There's sort of a silly argument as to how you could get mate. If a computer is playing black, it reads the board as greatly to its advantage. So when it's coming up on the 50-move draw, it might offer you a bishop to take to prolong the game. It has such a material advantage, a draw "feels like" a bad outcome, so it's worth giving up some material to continue the game and go for the win. Which is a very computer way of "thinking". Once you take all 3 bishops in this manner, you promote the pawn and get checkmate.