There really isn't too much to go on as to why this all happened. The video starts with the two being separated and it all spills onto a counter. As the customer is yelling for help and to get the security guard away from him, the guard gets close enough to land a few punches.
This is disturbing on so many levels. A special needs student was slapped and allegedly beaten by a security guard at Oakland High in California. Most of the incident was caught on tape, but victim Francisco Martinez claims he suffered abuse prior to what we see on the tape.
I don't think you need a video of a fake security guard hassling people to know that comic books nerds really don't like it when you try to keep them from going to or enjoying Comic-Con...but the problems it causes are priceless.