futures + debt based investing are the two canaries of market crashes. It was true in 2008, 2000, 1987, 1929, 1873, ... heck all the way back to tulip mania or arguably the financial panic of 33AD.
The question of "who's buying, how many of them are there, how long do they intend to hold for, and how are they buying?" you can't answer for sure, but when the capital to buy starts getting more scarce and people start leveraging other assets to continue buying...you'll get to a point where market velocity surpasses speculative velocity and the leveraging ability of the buyer.
In the physical world, this is a vendor of widgets on the street and a buying frenzy. Buyers spend money on the widgets and turn around and open up their own widget shop as sellers to feed the craze. Eventually enough buyers are converted into sellers, not enough buyers remain and the price crashes.
Futures contracts are a form of credit in this model. Bitcoin's crashed 4 times in 6 years - don't think it won't happen again.
