Well, "more people may be using HTTPS-Anywhere" may have something to
do with it.
Or, it might be that some new browser release just enabled HTTP/2.0, which
in many implementations *requires* SSL and might also trip this, as noted
in a posting on the topic which I just inadvertantly posted to this same
mailing list 5 minutes ago. 
Cheers,
-- jra
as to why your users just started it, nfi. my best guess is that they
weren't using https previously.Well, "more people may be using HTTPS-Anywhere" may have something to
do with it.
fwiw, https-anywhere doesn't just try to connect via https to every site you visit. there are rules that control where it will use https over plain http.
irs.gov and www.irs.gov are explicitly disabled, however, so it's not this.
cf. https://goo.gl/zTlzAu, lines 135-136.
Or, it might be that some new browser release just enabled HTTP/2.0, which
in many implementations *requires* SSL and might also trip this, as noted
in a posting on the topic which I just inadvertantly posted to this same
mailing list 5 minutes ago.
that's possible, i don't know enough about http/2.0 to comment.