A couple L.A. Chargers fans turned on each other during the Chiefs game on Sunday night ... whaling on one another's faces at SoFi Stadium -- as their favorite team played on in the background.
Video, obtained TMZ Sports, shows that a man in a Joey Bosa jersey appeared to have started the melee ... when he threw a couple of punches at a guy in a row below him.
Unclear what caused the altercation to break out -- but soon after the first blow was thrown, another man got involved .