Most of players rage comes from lack of game knowledge

if you give up in champ select due to picks you dont know how to play.

if you can only play one role you dont know how to play.

if you blame others for you feeding (unless some other lane gets stomped and you get 4-5 manned with no team help)

and half the time players rage about something they are wrong anyways, my team raged at me for not going to dragon when their team had 2 missing 3 dead i was the only one mid with half health and our jungler and bot was at base so i based and got there at the same time my team did and got it free still. Most of the time if you are raging about anything besides your play or 3 players extremely feeding on your team you are just a bad player who doesn't get the concept of the game.