Eh, I don't think he's that stupid. It was on purpose. Who knows why.
I agree with you, it's deliberate. He's waaaaay into personal nastiness. Remember his giving McCain the not-so-subtle bird during the debates? His nasty sniling at Romney during the second debate this year? The frequent displays of open contempt?
When Biden did that with Paul Ryan, I had an impression it was coached, that Joe's handlers asked him to do that. ("Go to the body, Joe, go to the ribs!")
When Obama does it, it's character.
Julius Caesar did it a lot, too, sneering and putting people down -- you can see these guys coming, if you know what to look for. Human nature wasn't invented last week.