Freo66, immediately before the incident, an Essendon player ran past and gave him a bit of an elbow to the body. A cheap shot. Mitchell reacted the way he did to this.
BTW, in all the vision I've seen, Mitchell talks about how inappropriate his response was, how he wish he didn't do it, and how it's not a good look, and blah blah blah. But unless I'm mistaken, he never actually said the words "sorry" or "apology". There's a big difference between thinking it was inappropriate and wishing he never did it and actually saying sorry for it.