Anecdotal, but: I suspect that the CBP have very specific criteria that can flag specific situations. I know someone who would systematically get arrested/interrogated by plain clothes officers if she was returning back to Canada alone. When not alone, no problem. Once, she was on the point of being arrested, when she pointed to her partner, and they stepped back. Although they never wanted to admit why.

After a bit of A/B testing, it seems to have been "older woman traveling alone with a fancy watch" that was flagging her. Without the watch, the problems went away.