The craziness of SPY has been unprecedented in past 11 weeks, at the pace of 0.5% incease per week to reach new highs from Sept. 12, to Dec. 12, 2019. The best run before was 0.25% a week from June 3, 2019 to July 26, 2019.
They are driven by a combination of 1) lower interest rate, 2) expansion of Fed balance sheet, and 3) improved world economy conditions. The major force behind the run is probably the market madness. The rapid surge in SPY price defys the fundamental gravity of SPY 500 index. Take an example of AAPL, the revenue decreased in last two years, while the company market cap has increased more than 50%.
