If you check sixfigureinvestingdotcom and read the May 17th blog, along with the comments and questions it might give some insight.
Also The Feb 17th article in Seeking Alpha, "Trading Volatility VXX and XIV..." by David Oldendurg (sp), along with those comments and questions also.
I have also noticed what you mention. What is different over the last few weeks is that VXX has been trading a little up and down but somewhat flat on a weekly basis. XIV has only been trading about 7 months, so what we're seeing now is how it functions in these conditions.
According to Velocity Shares advertisement, XIV is an accurate volatility ETN (with exception of fees). Therefore, I would tend to think that it would be a change of activity relating to VXX, which makes sense because of consensus that VXX is structured so "loosely".