Why do gas prices go down in the autumn??

Because school starts again in the autumn, and weather starts getting cooler and less sunny days, so people start to travel less. Demand goes down, so does price, basic Economics.

If the demand for something is low, then you can't charge too much, or no one will buy it. But if the price is low enough, than people are more likely to say "eh, why not" and buy it...even if it's something of "necessity" like gas. In July, when the weather is nice and you have no school, you're up for filling your tank up for whatever it costs and driving 2 hours to the beach any weekend. But in September, when school has started, your free time is limited and the weather is getting cooler, so you know there's no point in driving to the beach, so instead, you just hang out at home. If it costs $40 to fill up ($4 a gallon, 10 gallons) your tank and you can't guarantee good weather, and you barely have any free time, you're not going to bother to make that 2 hour drive to the beach. But if gas has gone down to $3 (only costing you $30 to fill up), then you may be more likely to fill up and atleast go out and do something, whether it's attempting the beach or driving to someplace closer for some entertainment.