There appear to have been no more articles giving any more detail, so there are several scenarios that are believable, but not provable (of course, some people will apply their political views to believe that their preferred interpretation is definitely the true one).
But here is an article discussing various issues regarding border checkpoints that those traveling internationally may want to know about: https://www.theverge.com/2019/8/31/20837448/social-media-dhs-cbp-surveillance-us-border-ismail-ajjawi-harvard . But also note that stuff like this could occur at border checkpoints of other countries.