I actually read somewhere that the British used to call it "fall" and we took it to America with us, but then there was a re-introduction of... classical ideas? Something, and "fall" was dropped in favour of "autumn" after a god or something.

...but just looking at the Wikipedia page, I think I may have remembered this wrong. :P But there was a thing about it dropping out of use in Britian but gaining ground in North America. So we did use to say it. :p