How do stocks crash?

Yes.............................

The value of a stock is only what someone is willing to pay you for it. The price goes down when there are a lack of buyers and sellers start lowering their sell price to try to exit their positions. If no one wants to buy it is worth nothing. It has nothing to do with other people taking your money. You buy at market value, the stock is worth $XX dollars. The bet is that it will be worth more in the future. If the company makes bad decisions and future growth looks doubtful, you are less likely to find a buyer at a higher price. You hope to sell for a higher price, but the value can go down just as easily. If someone manages to buy over 50% of the company's stock they can control the company. If someone decides to start accumulating shares to take over and the available shares decrease the value can rise. That is where the value comes from on most stocks that don't pay dividends.

it's all "supply-and-demand"

Prices of stocks will drop as soon as enough people BELIEVE that they are worthless

First off, the money doesn't just 'Disappear', someone other than you takes it as Their profit. Stocks crash for a reason, not necessarily a Good reason. It may be a shift in the economy, it may be a bad quarterly report, it is a myriad of reasons. 

Apparently I have lived thru 5 of them and my net worth has never been higher

https://en.wikipedia.org/wiki/Stock_market_crash