I'm about to win...but!

Why is it when I am closing in for a win, suddenly the opponent disconnect and auto resigns.

This happens when I am about to checkmate or I have gain material or a blunder from the opponent? Is this really the opponent just the other player resigning and the system not interpreting it correctly? 

They're not resigning and not continuing the match either. They're probably closing the app or storming off. It's bad sportsmanship. You can report it as stalling.

OH ok, I thought some of the games are good natured. It seems a bit odd as it happens a lot.

Etiquette gets better as your rating improves. There's a greater respect for each other and the game. It becomes more common to treat a match like a learning opportunity than a battle of egos. Not always though. Kasparov was prone to rage, for example.