Usually, fear. People hoard cash when they're afraid of a crash. When a crash happens, money becomes more valuable, so they can buy things more cheaply. People who anticipate a crash risk provoking one by removing money from the supply.

So central banks try to counter that: "Your money will be worth less in the future, so spend it now. Or make an investment that will produce returns greater than the rate of inflation." So they spend their money, creating demand, and jobs materialize (hopefully) to fill that demand.

For the last decade-plus we've had a combination of anticipation of disaster, and banks flooding the system with cash to assuage that fear. That's like pressing both the accelerator and the brake as hard as you can -- you don't go anywhere until suddenly something gives, and then the entire thing goes to hell in a handbasket right quick.

When? If I knew that, I'd be rich. The general advice is that the market can remain irrational longer than you can remain solvent.