Could have caught him off guard. He's watching the door or the crowd, and she sneaks up and grabs him. He may be unwilling to throw her off, in fear of injuring her, maybe even thinking she's just pranking him, until it's too late. I could see it happening.
I do not envy you the headache you will have when you awake. But for now, rest well and dream of large women.
Bingo. That’s pretty much what the video showed. She puts her forearm against his throat and pulls back while he stands still, arms straight to his side. If the cops weren’t involved, it just looks like another prank-video to me. Of course, she’s applying for law school and stands by the story she was defending herself (from her twitter).