What no one has mentioned is that the ongoing strikes in France had no impact on flights to CDG on the 9th. I took a quick look at several other airports including JFK/LAX and there were no cancelations. If France was metering flights they would not have canceled 1 of 2 flights from SFO. There were only 3 cancelations total which is normal.
I would demand to know why the flight was canceled and see if United will make you whole. The other interesting thing is that there were many options that day to fly to CDG from SFO via a one stop. Why was no alternative offered?