The score is thought to be dependent on a number of factors. I reiterate the word 'thought' as HubPages has and probably never will release any of its algorithms relating to scores and such. However, it is obvious that there are a few important factors that play a key role in the score as the algorithm must be logical. Here are a few of these:
Views - How much traffic the hub receives
Ratings - How does the community rate your work - firstly when the hub is published it goes through the screening process of being 'hopped'. If you are unaware, this is where Hubbers rate or flag new hubs. This will have an effect on the starting score of the hub after a few hours.
Comments - if your article receives a lot of comments then it shows that your article is a subject of discussion or grateful comments, which reflects well on your Hub.
Backlinks - I'm not too sure about this one and it ties into the views factor. HubPages do measure the amount of backlinks that your Hubs have so if your Hub has a lot of backlinks then it reflects well on the article but again I'm not sure that it ties in with the HubScore.
Hope this helps!