The guy wanted to go to the bathroom while the seatbelt sign was on, and the flight attendants wouldn't let him. He got angry and pushed one of them, and then went to try to complain to the captain. This was perceived as an attempt to break into the cockpit, and to make matters worse he was still holding the bottle of medicine (i.e. unidentified liquid) that he had been trying to take into the bathroom.

An article on the incident.