The explanation could be very simple. Obviously Digg's algorithm is extremely important to the company, and Digg had put together a long QA and approval process to ensure its integrity. A change to the algorithm may require 5 people to sign off, lots of analytics, gradual deployment, etc.
In the meantime, they run into some problems with the ranking, and knowing their own algorithms, they chose to game it (or tweak it) externally rather than internally because they can't afford to go through the official process. And the intention is that whatever they did would be eventually incorporated back into the main algorithm.
Unfortunately, some low-ranking QAs were assigned to execute the hack, and they are oblivious to the negative perception such maneuver would result.
And another reason for doing it in the first place might have been to artificially inflate digg counts across the board to mask the lack of site activity after the exodus of so many users.
This is plausible, but here's the problem I have with that.
If you look at the data, these accounts are not digging only preferred publishers.
They are Digging mostly preferred publishers, but also occasionally random blog-spam. (ex buzzll.com)
If Digg was behind this, why Digg random blog-spam as well?
No, if Digg was behind this I expect that would either make the accounts realistic by simply taking what an average user Diggs and replicating it a few times, perhaps with only preferred publisher diggs.
Any first-year CS student could come up with a script to create plausible user names, at least better than those in question.
The Digg team is not the best, but I have slightly more faith in them than this.
What I think we have here is someone who figured out that in order to slip in diggs for blog-spam, they had to digg some "top stories".
Let's think about this for a moment.
Let's say you're writing a Digg spam-bot.
You know in order to make your account look good you need to Digg some top-stories.
The easiest way to do that is pick some stories you know will be top stories...stuff that's always on the front page...e.g. Digg preferred publishers.
Let's say you didn't do this.
In this instance, you'd have a bunch of accounts digging only the same small set of blog-spam.
They would stick out obviously as a set, and digging only the same small stories.
But, wait...
Now they are not digging only random small stories...they are Digging top stories...so now they are part of a huge set of "normal" users, and their digging patterns look more like a normal user.
I think what we have here is simply a spammer who found a hole in Digg's automated account detection.
In the meantime, they run into some problems with the ranking, and knowing their own algorithms, they chose to game it (or tweak it) externally rather than internally because they can't afford to go through the official process. And the intention is that whatever they did would be eventually incorporated back into the main algorithm.
Unfortunately, some low-ranking QAs were assigned to execute the hack, and they are oblivious to the negative perception such maneuver would result.