I was playing a game of halo 5, but I had to go do something so I quit the game. After I came back I wasn’t banned, so I went and played slayer. When that game was done it kicked me out and now I have an 8 hour ban. Iike only quit one game, but bans me for actually 9 hours if you count the 58 minutes! Your game rocks and all but ever since the update it’s been striking ever since I quit 1 game.
You have at least 10 DNF matches in your account history over the past 10 days and more the further back I looked. You never get banned for just one game, but it generally will be the actions of 1 game that lead to triggering a ban. The system itsself generally runs a game or 2 behind which is why it can look like you are banned from winning sometimes. To reach an 8 hour ban you need to have been banned previously for quitting or idling in too many matches.
If you need to leave a game generally folk will be ok, IRL stuff does happen from time to time. When you find you need to leave often enough that you start getting banned, it might be time to rethink you time for playing. Maybe stop 1 match earlier so you don’t have to quit? Or where you can, wait out the last few minutes of the game before backing out to the lobby 
Feel free to move to the sticky thread in the H5 section.