With the other 2 players leading and tied at $12,300, it made sense for them both to bet it all to go for the win. Meanwhile, with only $1,000, Manny wasn't going to have a shot at outscoring his opponents if all three were to get the question right, so he left himself the thinnest safety net in the world, and it caught him.
Don't feel bad for Manny, by the way. He came into this game with $42,798. He'll be fine.