Autumn, also known as Fall, is the season the immediately follows Summer and precedes Winter, characterized by falling leaves from dead trees.
By rendering fitful as fitfull he refreshes a literary adjective: the wind is made more alive, somehow, by being fitfull – full of fits and starts. Similarly, as his eye favours the double l, his ear prefers the double s (gusts/shakes) even if it means disagreement between noun and verb