He failed to show up to a hearing about his green card. Not saying his treatment is warranted, but all those cases had at least some kind of merrit where an ICE agent might become suspicious. It is not as random tourists getting detained.
"Schmidt’s green card was allegedly tagged for him failing to attend a hearing because the invitation was sent to his old address, according to the family’s fundraiser to cover Schmidt’s legal costs and loss of earnings.
“To compound this error, he had just recently been provided with a new replacement Green Card since he had lost the original one,” the fundraising page says. “Even then U.S. Immigration failed to let him know that there was an outstanding hearing which he had missed and that his card would be tagged.”"
(It’s not clear to me if he has been released. The last media reports are from two weeks ago)