The Federal Reserve is printing $90B per day (on average) in open-ended QE. This liquidity is used to take agency paper and treasuries off the hands of primary dealers and hedge funds. In turn, the managers of these organizations refuse to sit on cash, so they grab other assets. Some (large) percentage of these assets will be equities.

Anecdotally, retail investors also appear to be buying the dip in volume, though we need more recent fund flows data to see if reality reflects anecdotes.