From what I understand in an economic downturn the wages are "sticky". This results in increased unemployment. To combat this the central bank according to monetarism lowers the interest rates and "prints money" (QE).
But how about the return on investment, is that not also "sticky"? As an example say that I see that I get 0% interest rate on my savings in the bank. I get upset. "Last year I got 3%" I may think. So I move my money into the stock market in the form of index funds. Saving in the bank is safe. I was guaranteed my money back + interest rates. Having my money in the stock market is less safe. The stock market may crash. So due to the "sticky" nature of return on investment I have increased my risk to maintain a certain percentwise return.
Say millions of people do the same. Since the index funds are market cap weighted they have to buy the most stocks of the companies with the largest market cap: Apple, Microsoft, Facebook etc. So the price of the most valuable stocks go up even further. As a result of this the pricing of these stocks in particular but also the stock market as a whole may increase to unsustainable levels and we could have a bubble?