A future priced above spot must fall to meet it if spot does not move. That decline is the roll yield, and for volatility futures it is unusually large because the curve is typically steep — several percent a month rather than the fractions of a percent seen in most financial futures.
This is why short volatility positions have a tailwind and long ones a headwind, independent of any view. It is also why short volatility is so seductive: the position pays you to wait, right up until the day the curve inverts.
Example: front-month volatility future at 17.5 against a spot index of 15. Over 30 days to expiry, with spot unchanged, the future converges to 15 — a 14% gain for a short position and the same loss for a long one, from doing nothing.
Related: volatility-futures, vix-futures-curve, etp-roll-decay, contango