Two reasons - Autumn is associated with food (食欲の秋 - Autumn of Appetite), and that you can cook them using the heat from burning the leaves that have fallen off the trees.
It could be seen as a reward for raking all those leaves up, even.
Foood!
Foood!!Autumn, like, somehow makes your tummy empty!w-w-wants to...Yuudachi... you can't go and bother Harusame and the others, okay?Even in the Autumn, you just can't be lonely or anything, can you.Fwah!!?
Yuudachi-neesan!?C-Commander!
Harusame wants...Wants to do her best together with the Commander in the 2ndDesDiv no matter what...
