It's unusual to see any opinions on a stay application, because the Court hasn't heard the case yet. Typically, you have a one sentence order that says "The application for a stay is denied." The way I read this is that the four liberals wrote an opinion dissenting from the denial, so Scalia felt the need to answer them, and Roberts didn't want to seem to be involved in the in-fighting.
Thanks for the analysis (from us non-court watchers).