I also vote that historically C- came from R-. That does not deny that there is some gray zone between MO and C.
A better question is – where did R come from? Usual response is to blame Haskala, Napoleon, etc. But why we do not see anyone from Bnei Brak becoming Reform currently? Because, over time, we developed counter-measures and defenses. We did not have those defenses when R started.
We had Rabbis denouncing them, we had R Hirsh trying to defend remnants of his community, but probably not enough. R Salanter thought that nothing could be done. At some point, he went to Western Europe, abandoning Lita, saying that it is possible to affect people who are at the bottom but not the ones who are still on the way from the top to the bottom. I am still not sure that there was no better way – modern yeshivas, not just one volozhin, chabad houses, etc. So, you can say that 18-19th century O- Jews caused, in part, Reform.