Actually you used to call it "fall" as well.
The alternative word fall for the season traces its origins to old Germanic languages. The exact derivation is unclear, the Old English fiæll or feallan and the Old Norse fall all being possible candidates. However, these words all have the meaning "to fall from a height" and are clearly derived either from a common root or from each other. The term came to denote the season in 16th century England, a contraction of Middle English expressions like "fall of the leaf" and "fall of the year".
During the 17th century, English emigration to the British colonies in North America was at its peak, and the new settlers took the English language with them. While the term fall gradually became obsolete in Britain, it became the more common term in North America.