Logically your hubscore should increase if you do all the positive things recommended by Hubpages - keeping on the right side of karma with links, comments, sharing, answering etc etc and writing original hubs with no hint of duplicate content.
It's very difficult to know because it's not an exact science but I suspect that content and links have some part to play, plus the amount of comments and sharing.
My score goes down to 94 sometimes- very quickly -then goes back slowly to 98 and I've no idea why! I try to write something daily on Hubpages - an answer, a comment, a vote up, a read, an addition to an old hub, more content to a not yet published hub.
The only way to find out would be analyse your every move on hub pages and try to connect your hubscore going up or down relative to those moves! Not an easy task.