This happened to me last month. On about the 10th of August I noticed that most of my hubs were getting 1 to 0 views, and this is even hubs that have been performing really well. Initially I thought it was a slump but I ended up with 0 views on all my hubs for a week at which point I panicked and went to Google to check it out.
All my hubs that were on page 1 of Google had not only disappeared off page 1 they were nowhere to be found on Google at all!
After much research and asking the community (there are several forums covering this topic) I took matters into my own hands. I went to each and every hub and did an update and republished them all (luckily I only have a few). That same afternoon my best performing hub that had vanished from Google was back on page 2 and within a day back to page 1. My hubs are now all performing steadily again.
I’m not sure if the updates and republishing is the solution but it seemed to help me. I hope this helps. :)