I would venture to guess it's falling traffic on the hub. The help page does say ratings are determined by quality of the hub and traffic. If you edit the hub, it will get a second look and will be shared again. This might bring up the traffic.
Per the help page (http://hubpages.com/learningcenter/Featured-Hubs):
"How the Quality Assessment Process Works
The Quality Assessment Process incorporates several factors, including:
-Ratings of a Hub’s quality as determined by people using the Hub Hopper
-Ratings of a Hub’s quality as determined by automated metrics
-Analysis of the how engaged readers are with a Hub over an extended period of time
As Hubs mature, are edited, and are expanded upon, they are reviewed again by the Quality Assessment Process. This means that any Hub may eventually become Featured, and vice-versa."