I think there are three possible answers:
1) Variation. The number of views per day varies from day to day. You need quite a few data points to be sure that a perceived drop is genuine and not just variation.
Solution- monitor for a number of days before panicking.
2) Competition. Is your hub lower in Google search results because some similar webpages have taken your position?
Solution- Use stats to see which keywords drive most of your views. Use those keywords to see if other webpages have now taken your position. Consider adding content or adding links to fight back!
3) "Google dance". When a hub or any webpage is new its Google rating seems to vary from day to day and week to week. It may then settle for months and then inexplicably drop.
Solution- Live with it! Write more hubs so that you are less susceptible to one hub's performance.