Well, I've verified that changing the temp directory has no effect on the issue. That's about all I'll be able to test until later this evening when I get back to my dev machine.
I will say that the problem manifests on different versions of Windows on different hardware with different software installed (as well as different versions of Sigil, Qt5, and QtWebKit). Some machines are virtual--with no kind of anti-malware/scanning software installed--and some physical. With the direct connection to Book View being open being the trigger, I don't think it's a third-party software interference issue, myself.
I remember something similar happening a while back where multiple image files (or all image files) were selected on Windows (when one or more of the images were PNGs). The problem then turned out to be Book View and Preview being open at the same time. The workaround was to temporarily close Preview if it was open (at the same time as Book View) and relaunch it after the delete if necessary. That workaround is still in place, though, and Preview is, in fact, closing when trying to delete the multiple css files (if it and BV are open at the same time). Plus ... I've made sure that Preview is not open at all when testing this new bug.
I can't help but think there's some kind of weird connection there.
EDIT: I think the workaround evolved from
this discussion. But in that case, Sigil was actually
crashing when PNGs were being deleted with BV an Preview open at the same time.