Real reason... we got into an unhealthy ether of leverage, debt, and risk.... trying to "get something for nothing". (Thank you Greenspan, Bernanke, Yellen) Oh... and that "the chickens always come home to roost".
#1 Fed been fighting deflation for decades and it's attempting to show it's self
#2 Low oil prices in actual and a leading indicator that puts pressure on equities prices
#3 Pandemic fears (regardless of how bad or not bad it gets)
#4 Markets were overbought, so any change in dynamic that isn't at minimal bullish, they will fill in the overbought levels as there's not enough bid to support the selling particularly when people are leveraged.
#5 Fed has been acting with very little effect.
#6 Presidential elections coming up which given current dynamic of candidates causes uncertainty to say the least.
#7 Technical damage to charts is so bad, that probabilities of even a 2-5 day rally holding without retracing majority of it very quickly are low.
I could point to any and/or all of those. I'd be more interested in hearing a case of why the markets shouldn't be going down.