I'm surprised that no one has actually commented on the knockdown at the start.. I do agree that it would be classed as a knockdown, and Joe genuinally lossed is balance from the punch (squared his feet up), then he fell backwards because he was off balance..

I almost thought for a second though that the ref wasn't going to score it a knockdown, because although hopkins did land the good punch, he also pretty much pushed Joe over as soon as he realised Joe was off balance...
I may be remembering it totally wrong, but that was my genuine impression of it at the time..

If someone could post a youtube of it I would highly appreciate it so I can see if my first impressions were correct of Hopkins helping him onto his back with a little forward momentum