Interesting. They were expected to move about the same percentages, however, seems a huge discrepancy. SVXY down 32%, while XIV only down 14%. I am also eager for enlightenment! -- Since I transitioned from XIV to SVXY about 2 years ago, I did not notice this, until you mentioned it.
You might want to do some looking back at the tape. These products in particular are susceptible to market forces moving them away from the indexes they're supposed to track. A big forced liquidation could make these move out of step...and today would be the type of day you could have them. Not sure how they (or any other VIX products) are structured, but if they're using different month futures, that would show (relatively) strongly on a day like today too.
Wow! Seems I may need to dig into the prospectus! My fear is more for a liquidity issue, than a lag in value representing what it should by a few hours after market close. -- Need more "senior friendly" products to trade. ;-) (Was waiting to go long SVXY, now need to insert some reading!)