All hubbers scores are spread across a range from 1 to 100. Your score may be going down even if you had not done anything and your page view remains the same. This is because other Hubbers may be writing good hubs getting the higher scores and bumping everyone else down.
Also, your score will decrease based on the length of time of inactivity or posting. A hubber who has not posted in one month will see the score go down a bit. A hubber who has not posted in three years will see his score go down by a lot.
This is just my guess, I could be wrong. I'm not a hub staff.