This happens a lot actually. I have a report on one of my games and the only thing that could even have been reported on the log was me saying "dam" and it was not even typed to the people who were telling me I'm trash and the worst jungler they've ever seen and I need to kms yada yada yada. You can type "you aren't doing well either, I'm 0-2 because mid and bot are in my jung while you lol'farming bot lane" and somehow that criticism right there is reportable.
You can get reported by someone who openly admits to trolling and feeding and griefing in your game and they are TRYING to get a rise out of you and when you type ANYTHING to that person that upsets them, they report you and then laugh because they see it pop up that you got a chat restriction.
More investigation needs to go in to things like this, if someone gets banned from your entire team reporting them and you see it pop up on the instant feed back, I feel like a report from said player should hold no grounds.
EDIT: Hell it seems like even telling someone they are cancer and getting reported because they are griefing soooooo much and won't stop flooding the chat with nonsense, it toxic in as well and gets you banned as well XD. Makes zero sense to me. Telling someone they are toxic is toxic? Riot acts like the un interested or lazy dad that just yells at all the kids to all go to their rooms, regardless of who's wrong in the situation Lol.