I can't imagine that it was a mistake. I'm guessing foul play. ...probably a prank. I doubt there's a "football to porn" switch. I've watched a lot of TV in my day, and it's never turned to porn on me.
They have the ability to watch things inside the station that are not being fed over the airwaves. All it takes is a flip of a switch to change what is getting fed. It very well could have been an accident, but either way the guy is going to have to explain why he was using company equipment for porn.