The market just didn't go up anymore and for the next 20 years it did badly. The 1929 crash reversed almost completely by 1930. Nonetheless, it's the one-day events that change the way people think. They become the symbol of something wrong. After the 1929 crash the market just spiraled down over a period of years, as negative factors became prominent.
Public opinion can change quite sharply in a few years.
Right now we're at a record high in consumer confidence. I don't expect that will continue forever. We're in an experimental period were we've got a lot of start-up companies with no profits, built on hope. One kind of event that could trigger a market decline would be the failure of a lot of these. Or it doesn't have to be that severe.