Just like the warmest time of day is not normally high noon, but
mid-late afternoon, the tropical waters which a hurricane derives its
energy reach their warmest in the late summer, even though the amount of
insolation is decreasing. Most surfaces, especially water, take some
time to warm in response to the increase in solar radiation. Also, the
upper level winds tend to be more favorable for tropical cyclones in the
late summer into fall.
I hope this helps.
