Shorting VIX has been a hugely successful trade post-crisis, because of a general belief that real volatility will always be absorbed by the state, so there is no real risk associated with collecting premiums from selling short-term volatility. A lack of willingness to sell vega longer term on the curve - has kept the curve over-priced on a relative level.
As the time of the settlement approaches, an imbalance of shorts routinely hits the market (which got exacerbated since '11-'12 due to ETPs). Too many willing sellers of short-term volatility come up against not enough buyers in the adjacent contracts. As the shorts compete even more aggressively for a chance to stay in the business of selling volatility, downside pressure mounts on the next set of short-term futures.
Of course, the curve compression eventually becomes far too expensive for many shorts. They are then faced with the choice of either moving out further across the curve (and flattening the short-term curve in the process) or with pulling out of the market completely. In the latter case, that means closing the short at settlement, something which puts upward pressure on spot/next contract.
The current, short-term/low-VIX environment, is associated with selling of as much of short-term VIX as possible, which would be (already is) followed by the over-selling of mid-term VIX, and eventually - by the full scale selling of volatility across the entire duration spectrum. The flatter the short-term curve becomes, the larger the incentive to pull out of the trade entirely or to sell volatility further up the curve, where a positive roll yield might still be achieved.
The rise of VIX ETP products and the 'btfd' mentality, which has worked tremendously over the past...idk how many years...has led to a generation of 'vol-tourists'. They focus on buying 10-15% dips in inverse ETPs and selling the UVXY calls. They are clearly unfamiliar with the self-enforcing type of convexity, associated with these products: the more the VIX rises, the more shorts must cover, the more levered ETNs must buy to re-balance, the more volume we get, the more we interpret panic, the more mispriced the curve gets, the more shorts come in (which didn't get squeezed yet, thanks to btfd).
The VIX complex doesn't exist in a vacuum either. Not sure, if the VXX prospectus was updated since its inception, but back in 2010 "...they didn't expect to affect the SPX IV...":
"
We or one or more of our affiliates may hedge our obligations under any series of ETNs by purchasing or selling equity securities underlying the S&P 500 Index or listed or over-the-counter options, futures, swaps or other derivative financial instruments; the S&P 500 Index (including the put and call options used to calculate the level of the VIX Index) and the equity securities underlying the S&P 500 Index, and we may adjust these hedges by, among other things, purchasing or selling any of the foregoing.
Although they are not expected to, any of these hedging activities may adversely affect the market price of those items and, therefore, the market value of the ETNs."