Do you remember 1973-1974, when the S&P 500 index plunged 50%, even while earnings increased by 50%? That was fun. Hussman writes more here.
What "happened" was more or less the same thing as today. All variables, market levers, were cranked the opposite direction when the bull ended compared to when it started. Consequently, earnings didn't matter. Nothing needed to happen to cause a crash.