There are a number of possible reasons for this.
For example, the last quarter of the year is the biggest selling season, and so it's likely advertisers have increased their ad budgets and what they're willing to pay.
Secondly, it depends upon what particular hubs are getting a traffic boost. Some topics attract a lot more visitors, but aren't conducive to big advertising dollars. So if you get a bump in traffic on those, it could drive your CPM down.
Having said that, there have been times I've noticed where the CPM drops so much it appears to be a little unrealistic for those who work hard to make a little extra money. The worst I've seen has been when CPM has dropped from between the 2 to 3 range (for the overall group of hubs).
It would be nice to have Hubpages management explain, beyond the couple of things mentioned above, why it can drop that low.