I'm not a financial...well, anything, but it's my interpretation that most large stock market drops are due to massive selloffs. Basically, people who own the stock are trying to unload it before something bad happens to those stocks. The result is something bad happens to those stocks, and things start crashing.
Like in Sneakers:
People think a bank is financially shaky
Consequence: people start to withdraw their money
Result: Pretty soon it is financially shaky
In other words, stores running out of things and/or online sales being strong aren't, by themselves, the cause of major market movements right now; it's speculation about what'll happen tomorrow, and making sure you don't own any of the stocks if & when it tanks. Which incidentally causes the stocks to tank.