The trade war is the most likely catalyst for a downturn - and it's a pretty good one, since it seems clear that the USA-China conflict will endure and not just be resolved overnight with an announcement.
Otherwise, look at the ongoing growth in passive investment vehicles, the huge explosion in SWF assets from <$1trn fifteen years ago to at least $7.5trn today, and the increasing concentration of the world's wealth into the pro-managed family offices of a few billionaires. These market players hold for the long term, allocate via systematic methods, change allocations only very slowly, and supply a constant bid for shares from new contributions and recycling of income.
This is a recipe for what we've seen for a decade, i.e. a steady grind up punctuated by occasional washouts of trend-followers, crowded quant strats, and individual sectors.
It's possible that U.S. stocks specifically could go out of fashion and just chop sideways for years or decades, but that's unlikely unless (until) the silicon valley unicorn tech-disruption macro story is decisively crucified.