Apparently, it goes back to the end of a season back in the 70s. They were both battling against the drop going into the last game. Both games were supposed to kick off simultaneously but, because of traffic, the game involving Coventry kicked off 15 minutes late. Which meant that Coventry (and their opponents; Bristol City, was it?) knew that if their game finished as a draw, they'd both stay up and Sunderland would go down. Which duly happened. I'm assuming that Sunderland think that both Coventry and Bristol basically downed tools in their game.