There are probably too many factors to really narrow it down without more information. Could be that google made some changes. Peoples tastes are fickle, could be they aren't searching the criteria that matches your hubs. Are they all relatively new hubs, because your views could be skewed by those reading them in the hub hopper.
Coming off the holiday season its normal to see a drop depending on the types of hubs you have. Also, with warmer weather people are spending more time outdoors than cooped up inside.
Really too many possibilities without more information. Try using google analytics to see what search criteria people were using. Or you can add a sitemap to help increase traffic.