Here's what Hubpages help has to say.
HubScore takes into account many factors and will change over time as data about its performance continues to stream in. It will also fluctuate quite a bit, seemingly at random, and that is nothing to worry about. The most important HubScore components that you should be concerned with include:
Amount of traffic - including percentage from reputable sources other than HubPages
The reputation of the Hubber - your Hubber Score and contribution to the community
The response of readers to your hub - including comments, thumbs up, etc.
The uniqueness of content - copying content already available on the web will be penalized
So it would be High Quality, Original material which receives high traffic. Being actively involved in the hubpages community will also help.