How does leavebuster exactly work?

I got disconnected from a game around 1 min mark, then I got a blue screen, don't know why, doesn't matter. My team decided not to remake, so I came back around 8:30 min mark played the rest of the game, around 40 min, and after the game, I got a leavebuster penalty. My question is: why does it work like that? I played most of the game, more than 80% of it, but still, I got put in a low priority queue, sometimes I see people disconnect in the middle of the game, afk for quite a long time and when the game is over they're not marked as afk. Why is that? Can someone explain how does this work?