I've recently experienced automatic forfeiting while winning (not at the end of a game, but right in the middle, sometimes with ball in mid-flight). Near as i can figure out, it happens if I get a request to play from a friend that I don't "reply" to. If I just let the request time out, it would appear your current game gets forfeited. I now reply as soon as possible (indicating that I'm in a game) and haven't had this issue.
Something similar happened to me while I was leading with a bud in an Alt-Shot. We were ahead and the game kicked me mid-flight of a ball. I couldn't get back in fast enough and we lost.
This shouldn't happen because of another invite to play though because the game should turn you "red" when you enter a match, Alt, skins, or Blitz though. Not turning "red" would be a WGT glitch.