I've read Something Wicked, Medicine for Melancholy and October Country and I'm curious about the significance of Bradbury's fixation on Autumn. There IS a repetition of this theme, or am I over analyzing?

I wouldn't say so. While you're at it, there's an even more marked and dramatic underemphasis on winter. Perhaps the forthcoming "Dandelion Wine" sequel, "Farewell Summer," now at the publisher's, will shed further light on this.