Showing your co-authors the manuscript for the first time a few hours before you submit it to Nature is not how it's done, not at all. That is really unusual an

Showing your co-authors the manuscript for the first time a few hours before you submit it to Nature is not how it's done, not at all. That is really unusual and a giant red flag, though the PhD students were probably not experienced enough to judge just how weird this was. And I could imagine that in that field the PI could convince them e.g. by stating they wanted to avoid getting scooped or something like that.