It will go back down by playing without leaving, dc'ing, or other types of AFK behavior. It just takes a lot of games (far more than just 10).
They system is partially based on total games played, influenced by recent behavior.
You're at level 14. That means you have maybe 100 games played? Less? That's an absurdly high percentage of games if you've left if you have already triggered the 20 min level twice.
Don't ragequite, don't play if you know you're likely to get interrupted, don't play on a questionable internet connection, and don't give up and dance at the fountain or whatever other kind of movement you might try to dodge the auto dc system.
Whatever reason you have for leaving doesn't matter, just don't leave, and you'll eventually get your account back to a good standing with LeaverBuster.