@Frank Re: @taken handle...
You're right, the CBP knows why the flagged him.
But then so should he...
Something he said. Some place he traveled, who he met with...
His political views. His relatives.. A whole slew of possible reasons.
Or it could have been random and he said something to piss them off.
I've never had an issue w CBP in the US. In the UK? Different story.