If anyone is interested in this sort of thing, I'd highly recommend the (very technical) book Why Stock Markets Crash by Didier Sornette - a guy who comes from a Physics/Geophysics background.
tl;dr - Stock markets crash because of fractals/chaos/complex systems. And there's no such thing as an equilibrium (in the stock market) because if there was, everyone would agree on the true value of a stock and no one would buy or sell for that reason.