hk2 ETFs are a form of Para mutual betting. The inverse ETFs make a bet against the twin ETF long. That is the primary source IMO. The prospectus clearly states that in market conditions that are not favorable to maintaining a 1to1 or 2to2 relationship the fund will perform as best as it can but does not make a guarantee. Probably what you will see is the spreads widen between the two opposing ETFs depending on market conditions. The above is my interpretation from reading the prospectus.
There is a demand factor in ETF's. Witness DUG vs DIG. Demand destruction or people driving less out weighed hurricane worries. A little common sense goes a long way. If one is not using these little gems, one is the loser. Para mutual BS be damned.