In the hots casual 'lobby' aka quick match, you often don't have enough players of similar skill level, so the match maker tries to create balanced teams with quite a large mmr spread. There are many ways this goes wrong, but the end result is your winrate tends towards 50% because that's what the match maker is trying to do. It performs very badly at this and so you can have big streaks and bad quality matches for an extended period of time.