I don't think this has been established... in some circumstances, waiting (a silly amount of time) and the game continues.  But in other cases, the game appears frozen and won't continue until a player leaves or 2... etc.  So, its some sort of connectivity issues.  As the game uses p2p, it could be a problem with individual players connecting to each other... the time range that the issue occurs appears to be random.  I personally think its something that got broke 1 or 2 patches back and is related to the way auto-kicks should be handled.  I think its a case where 1 player should be dropped, but it never seems to happen.  This is generally infrequent, but does tend to happen... if you find it happens more often than 1 in 50 games, it could be an issue on your side.