Someone that knows more than me: how is a correction going to happen on index funds?

Retail investors are told to shovel money in and keep it there. Who's going to be selling to pop the bubble? Do investment banks have a lot of index funds and their derivatives bought? Does there have to me some major re-allocation within the fund that causes investors to sell?

If a single stock is valued incorrectly, how is that going to bring down the entire index?