Why do fuel prices drop at the end on Q3 every year before cola for Social Security is determined?

[Son of Nun]

I have noticed the change in gas prices dropping nearly 20% from mid August to the end of September each year, just before the cost of living adjustment is made to Social Security payments for the following year. Since fuel costs are included in cola, how is it that those costs go down and minimize the adjustment for people who are on limited and fixed incomes? Are there actual market forces driving costs down in the middle of hurricane season, or is this is planned and coordinated attck on beneficiaries?