The comparison he gives is that of a theater which keeps getting more and more crowded but the doors remain the same. In the bull market everyone keeps buying m

The comparison he gives is that of a theater which keeps getting more and more crowded but the doors remain the same. In the bull market everyone keeps buying more and more index ETFs which are highly liquid. However, over half of the underlying securities have low trading volume. As long as investors keep piling into the ETFs there is no problem. But when investors start selling the ETF, and the ETF has to sell the underlying equities, then prices may fall much faster due to low volume and no demand.