It all went down on Saturday at the bay area hotel where the team had been staying since playing the Raiders in Oakland the previous weekend.
After the players and coaches had boarded their buses to go to the airport to head to Seattle to play the Seahawks ... Rivera got off his bus, went to Cam's bus, and pulled Cam off for a 1 on 1 convo.
We're told this was the moment Rivera confronted Cam about his dress code violation -- not wearing a tie on travel day.
You can see the look on Cam's face ... he's obviously disappointed and retreats back to the bus.
Cam was benched for the first series of the game on Sunday as punishment.
After the game, Cam explained, "I didn't follow dress code, and Coach told me I wasn't going to start. I stand by his decision."
Cam said he had neckties available -- but the reason he didn't rock one? He didn't feel it went with his outfit.