A lot of people like to point different fingers to what caused the market crash, but I think the answer if right in front of us.
It wasn't subprime mortgages, it wasn't bankers, it wasn't Main Street, it wasn't leverage/derivatives/CDOs/collateral/tranches:
The SPX was hitting historical resistance (a.k.a Supply levels). Usually, when we reach thise historical levels, there is more supply than demand which casues sellers to come in. I think this is why we saw the market drop, there weren't enough buyers to break that key supply level.