I visited the US last year for a conference (from the UK) Border Guard was very angry at me for some reason, very cold, didn't smile. Quite curt with his respo

I visited the US last year for a conference (from the UK) Border Guard was very angry at me for some reason, very cold, didn't smile. Quite curt with his responses. Which was strange because he was laughing and joking with the couple before me in the queue. He took particular umbrage with the fact that I'd ticked "Business" on my customs form, as I wasn't sure what else to tick - I was attending the conference on behalf of my employer who had paid for my trip.