I don't think the weather has changed enough to be a factor, if anything, folks are outside more now than in August one would surmise.
I'm back to "area under the curve" notion again, partially anyway. France is in serious trouble and there is little government can really do. The imposed a curfew 9 PM to 5 AM which might do a little, but the thing is out of control there. Will they shut everything down again? I don't think so, I think they will have to muddle along with selective closures and repeated admonitions. They could swamp their hospitals soon.
The wife says this surge is because of vacationers in August and younger folks partying, which is plausible I suppose.
Sweden is rising as well, which tends to give lie to the herd immunity notion. They are reporting 800 or so new cases per day (France is 30,000 on 6.5x population base).
Something has changed, and I am not clear what it could be really other than behaviors. School? People going back to the office?