It appears we need to log in and contribute in some way pretty frequently otherwise our Hubscores go down. Like many of you, I've noticed my scores go up and down without apparent reason. However, if I simply go in and answer a question or two, or partake with answers in the various Forums, the score climbs a little.
I have an awful lot of Hubs placed and suspect that even the number of Hubs one has published makes a difference. When I started off three years back my score was, on average, lower than it is now. Now it seems to vary between 96 and 89 no matter what I do.