CBP will never tell me, but I'm pretty sure that's why. Started when I was traveling with a group. Has been happening to me ever since. Like I said, they ask me the same questions every time, but they usually just do it at the kiosk. This was the first time I got detained.
And it was detention, because I was locked in and absolutely NOT free to go. CBP doesn't call it detention. Don't asked me what that's about.