Last crash of NDX provided a clue of the condition needed to stop such B.S. rally - when the hope is met with reality.
Using the internet dotbomb as example, when those companies started to finally produce earnings, but staying at very low level (almost none), making P/E finally showing a number instead of N/A, then, long term funds are FORCED to unload those stocks as they cannot hold something at P/E > 100 over 2 consecutive quarters.
Simple as that.