For the last decade many of the largest corporations have been taking on massive debt loads in order to buy back stock and inflate their stock price. This is a result of extended low interest rate policies. The corporate "logic" is, "we are growing at 3%, we can borrow money 1% now and take that cash immediately, and since we are growing faster than the interest we'll have no problem paying it back". So they buy their own stock back, and their stock price rises. And when many corporations do this, all of their stocks rise and the market rises, and rises, and rises - like its been doing for the last 10 years. At the same time, debt grows, and grows as the FED scrambles to keep cutting rates so that companies can keep borrowing and allowing for cheap money so that this stock market rise can continue. Then, eventually, there is a period of time when they dont grow at 3% - worse yet they shrink (like during a pandemic). Then, all of a sudden, they are left with trillions in debt that they cannot service (debt that has been collateralized into bonds and sold off to other investors, which will also default). That results in a cascade of asset selling to raise cash, which tanks the market, which results in more selling and you have a crash.