Summer is fading, leaves are starting to scatter the ground and the weather is turning chillier. It's not yet time for winter warmers, but crushed ice-charged...
In North America, these cocktails are still 'autumnal'. It's not like courgette vs. zucchini where you'd probably get a blank stare if you say 'courgette' to most people (in the US at least), both 'fall' and 'autumn' are used to describe the season in North America. 'Fall' has the benefit of being shorter, which likely leads to it being more common.