It's impossible to specifically analyze what happened to your traffic from the information you give. What was the traffic source for your affected Hubs? If your traffic has slowed down from search engines, then it is due to faulty SEO (search engine optimization). If your traffic was from social networks, Pinterest, forums and the like, the interest has run its course and you'll need to find new avenues for traffic. If most of your traffic was from Hubbers, once your followers have read your Hubs the only other internal traffic you will see is that from the related Hubs section at the bottom of other Hubs and HP has recently changed its algorithm for determining which Hubs to show.
Hubs in the Editor's Choice program have been reported as losing their traffic for some time because of the change made to the URL of the Hubs. This traffic may or may not return to its former levels.
I would advise you to check for problems with Google in your Webmaster Tools account and see if any penalties have been applied to your subdomain.