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.
Can equilibrium not exist in a dynamic system? Think vapor pressure - the particles can agree on temperature but still be trading positions between gas and liquid.
Dynamic equilibrium is absolutely a thing, but it’s fiendishly hard to tell true dynamic equilibrium from dynamic metastable equilibrium. I don’t think anyone has claimed that economic equilibrium is ever anything other than metatable, and we always seem to discover new and terrible ground states.
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.