For me the change in weather really saps my energy during the autumn. I'm not sure what, but more than anything else the shorter daylight does something to me. And the cold sucks the energy right out of me. I found out I need to eat more before my morning ride, now that it's cold.
Then I pick up once the winter Solstice passes and the winter brings some bright, crisp days. But autumn, man, less and less light, colder and colder, wet and windy - ugh. Nowadays before I bike I have lots more clothes to put on than before, too, and that can be a bit of a hassle.