It seems to track economic recovery, seeing the historical growth largely coming after major recessions/depressions. Such growth may be the intersection of people starting to be able to invest again, but still having anxiety related to the decline of the business environment they just went through, thus not wanting to invest in businesses.

I wonder if there is concern about the incoming boomer peak selloff, i.e. the larger than proportional age group coming into retirement and about to (presumably) sell off a bunch of indexes/stocks in their 401ks. Not sure what else could be a good hedge.