The supposed "crime" dozens of people were jailed for: Voting among themselves which candidate should run to represent the alliance. A caucus, basically.
https://www.barrons.com/news/benny-tai-from-hong-kong-s-elit...
Back when I was still there in 1997, I didn't expect the PRC to break the treaty-promises quite so quickly and blatantly. (Many factors have changed since then.)