Disconnect Early in game multi times. But Win game.

I just had a game where I am getting disconnects rapid sessions at the start of the game. I restart my machine and I am playing with no problems at 9 mins into the game. We win the game, but I get a LEAVER and get demoted. Can you explain how you cannot tell a disconnect from an AFK? Also, can you explain to me how I can get punished for a DC when I finish the last 25 mins in my 36 min game with no DC? I even have a screenshot if that helps.