When the momentum of short-term returns attracts enough money that the makeup of investors shifts to mostly short-term.
As more short-term traders are attracted by the momentum of the asset, the valuation of the asset keeps going up - feeding the momentum.
This leads to over-valuation of an asset and hence, bubbles forming.
When do bubbles do the damage (burst)
When long term investors fail to understand that short term traders are playing a different game than them, and start taking their cues from short-term traders