In a high-pressure situation they sometimes fall back on their quaint expectations of rights and basic human respect. I would wager that someone who had spent serious time in prison would do much better in these situations.
The guy probably had a long day. I bet he just wanted nothing more than to get home and go to sleep. He obviously didn't understand what all the fuss was about, and he ended up stepping on someone's hair trigger. It seems terribly unlikely he did something that you or I would consider physically threatening. After being pepper-sprayed and man-handled, and likely not in the soundest state of mind, maybe he even flailed a bit -- guaranteeing an assault charge (and possible conviction).
Perhaps that's how it went down. It'll probably turn out that the border guards "followed procedure." But that's exactly the problem that makes the story frightening to us:
The path from "long day + non-violent temporary failure of judgement" to "being physically assaulted and facing a life-altering felony charge" seems terribly short.