Well, personally, I subscribe to: https://en.wikipedia.org/wiki/Random_walk_hypothesis

I won't mislead you by pretending to be an economist or expert, but you should look up the "business cycle". As the name implies these booms and busts happen pretty periodically. In the past it's been approximately every decade.

As for the magnitude, I can't really speak to that much. Perhaps someone more knowledgeable will chime in.