Crashes typically happens because some part of the market was overhyped and people invested too much money in it. But what happens when the market as a whole is hyped, and people invest lots of money in that? Well, then the market as a whole becomes overvalued, and the market as a whole will crash in the next recession. You can see this being the case with the buffet indicator, stock valuations are higher relative to gdp than ever before. So instead of people pumping up some specific segment of the market that then crashes, this time people pumped up the entire market.