Do you mean they flag, or abort/quit the game? I have played a couple of games where people have made no moves for the last x minutes and flag because they blunder.
Why do people disconnect when I'm winning?

Mostly because they get frustrated when losing and decide to just abandon the game. Although sometimes I disconnect too when my wifi is bad so maybe don't just assume your opponent is tilting

It's because they can't stand the fact that you're winning, so they disconnect so that technically you didn't win for them.

Do you mean they flag, or abort/quit the game? I have played a couple of games where people have made no moves for the last x minutes and flag because they blunder.
Neither, they just outright disconnect.
People always seem to disconnect when I'm in a winning position... I like to imagine they're playing on a computer and they just yoinked the ethernet cable. Can somebody tell me what's actually happening?