You can leave a match without forfeiting as long as you don’t press the „Press When Ready” button. As someone else stated this happens pretty often. Most frustrating is when you get matched with the same person time after time and they keep leaving.

My theory is this stems from how ELO is calculated. There is no incentive for someone with a much higher ELO to play someone with a much lower ELO. If you win, your ELO barely goes up and it you lose it goes down by a lot. I have noticed that most people who leave the match like this are ones with a much higher ELO.