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1/ BTC 1mth ATM Vol continued to drop these last couple of days. 
2/ This happened despite a pick-up in realized volatility, with spot falling more than 12% in the past 5 days.
3/ In comes the skew effect: at-the-money implied volatility is lower in part BECAUSE spot has been falling. Indeed, skew is negative and therefore ATM Vol mechanically drops as spot rolls down the curve, everything else being equal.
4/ Moreover, this phenomenon is compounded by the fact that the entire curve has experienced a parallel shift lower. For instance, the Sep19 10,000 P is currently marked 11 vols lower than a week ago.
5/ All in all, not a great period for long vol positions as realized as barely broken-even lately, which means gamma P&L has not been enough to offset vega losses.
