That's what folks are theorizing. And it would certainly explain the lady in pink, regally waving to the office workers before the blood starts splattering. And, of course we know that the the big event is coming. Last week, we got a glimpse of a wedding invitation (November 23rd, 1963!) and everybody in America groaned, "That's not going to go well!"
According to Pandagon (via Jezebel): "The Great Foot Mangling is the JFK assassination in miniature, though of course it’s an accident in this case. But all the elements are there: Guy McKendrick is the charming young leader taking over from Boring-But-Efficient. People are initially wary of him, but his charm is hard to deny. A sense of cautious optimism kicks off the party, and everyone really starts having a good time, until a sudden and shocking act of violence blows through it all. You even had Joan walking around in a bloody dress hours after the event, just as Jackie Kennedy was. And suddenly you realize that nothing will be the same. For Sterling Cooper, the 60s are really underway now. Of course, we saw that Joan is the only one who didn’t collapse under pressure during the Great Foot Mangling... Watching her cry was tough, but the whole foot incident should erase that from everyone’s memories, giving her a chance to ask for her job back without sacrificing her dignity.And for those who are skeptical about my theory about the Great Foot Mangling being a tip of the hat to the JFK assassination, I offer a comparison. The shot of the vehicle moving through the crowd, the slow turn, the movement for a bit, and then the explosion of gore."
The Mad Men scene is HERE. The Zapruder tape mimicking the John Deere's turn is HERE.