WE GOT LEAVER BUSTER FOR YOUR GAME CRASHING?

I'm afraid, by definition, it is your game (as in game client on your end) that crashed, not theirs. You're the only ones not in the game if your game crashes, whereas if their game crashes then there won't be a single player in that match and riot has almost always issued loss prevented in those cases.

Sure, you can say their game's poorly made and failed to properly lead you into the game when there's no issue on your end. Regardless, it's still your responsibility to make sure you'll be able to connect and fully participate in a match before joining a game involving other players. If you believed you could because this issue rarely happened in the past then don't be bothered and just keep playing. Things happen since nothing's perfect apart from ideals.