The shift from Summer to Fall is traditionally one of the more notable seasonal changes in the stock market. We're coming into tax planning season (not usually too much effect on stocks that are up). Grand strategy decisions seem to be more frequent (High Holy Days? Memory of start of school year? All the needed people are back from vacation?).
I remember 1987, and this doesn't feel like that at all. Biggest risk is that DC budget chicken will turn into a wreck.