It's probably due to the change in URL for all Hubpages accounts. I've noticed many of my hubs climbing up on search engines off late. However, this doesn't mean that there would an increase in ad CTR.
Honestly speaking, the ad placement on Hubpages is terrible. It damages CTR tremendously. After having experimenting with ad placement on over 30 sites of my own, I must say that the placement of the first ad box here is just illogical. Since that's the one with the higher CPC, most people ending up only a fraction of what they could do with better placement.