If you were toxic in postgame chat, that is most likely why you were recently banned.
If you got banned and you think it was for no reason but you were actually toxic in postgame chat, that is why you were banned. The myth of postgame invulnerability isn't true.
I've seen enough evidence to convince me that this is the case (my own ban experience along with that of another guy I played with/against, with postgame toxicity being our only common ToS violation).
Regardless of how much people complain, it still doesn't make "Riot bans people unfairly" a true statement whenever a ban wave happens.