Stockfish trolling

So I was playing an automate game, and my setup was doing well, and near the end when a pawn was about to promote I was just thinking "oh well, good game".  Nowhere in my mind did I even think about it promoting to a queen.  But Stockfish is a robot, and with the mundane, clinical picking apart of the game it does, it can get bored.  
Although I found it a bit funny that stockfish promoted to a bishop, I do wonder why, and not a queen?

I just realized the game doesn't work but it doesn't really matter

Huh. Strange.

Stockfish will underpromote for 2 reasons.

1) Stockfish knows the piece will be sacrificed. If you have a choice of borrowing $5 and paying it right back or borrowing a million dollars and paying it right back, stockfish will always choose the $5, even if both choices are "equal", because it prefers losing $5 over $1,000,000.

2) Engines look at "candidate" moves. They don't look at every move, they select the best moves and choose from that list. Taking a queen is always going to be a candidate move, but taking a bishop may slip by. The other engine might not think it "worthy" and stockfish may be able to keep the promoted piece.   

huh, that's interesting