Border guards are trained to make you feel uncomfortable, because they need to be driving with you to in a reactive state. They adapt to build psychological pressure. Don't get too hung up on them "singling" you out or targeting you, a lot of it is carefully choreographed and rote.
I've also heard from a cabbie that crosses the land border often that sometimes the guys in the booth will send someone inside, then come inside and request to deal with that person themselves. This allows for massive amounts of abuse for assholes on power trips. If it has to go through multiple people, there's at least a chance for one officer to say, "WTF" if another is out-of-line.
Some people are super-nice, and others are complete assholes. This is not by design. If it was all choreographed, then the experience would be very much more homogeneous.
That said, there is a difference between a 'stern line of questioning,' and making an ass of yourself. Conversations like:
Officer: Question1?
Me: Answer, because...
Officer: Stop trying to qualify your answers! I'm asking
the questions here asshole. Just answer *exactly* what
I ask you, are you stupid or something?
Later I found out its a common ploy they use to illicit an emotional response and they watch for tell tale signs that I would give myself away if I was lying.
Unfortunately for the lady the holiday was roughly a week of heavy drinking on yatchs and partying all night, so I was absolutely exhausted. So I let out a rather unrepeatable hail of abuse at how stupid she was and the poor woman didn't look up to meet my angry gaze and just stamped my passport... and I walked off nervously, full of guilt, expecting a security guard to escort me off for questioning or something but no one did thank god.
So they have all sorts of things up there sleeves to try and read your intentions from the get go, I've never had any abuse myself though, but then I've never travelled into America before either.
Case in point, my co-workers and I would always have difficulty going through US customs (which, is located in our Canadian airport) when questioned by one specific agent.
If we go through US customs and get any of the other agents, 9 of 10 times there would be no issue at all.