One Digg user has done some data mining using Digg's API and found "159 dummy accounts all have fake names (such as 'dd1' pictured) and seem to only have contributed to submissions from Digg's publishing partners to get stories onto the Digg front page with almost no activity from real users." The suspicious activity began after an algorithm revision that took place on Oct'15.
The methods used to collect this data and the data itself is available on Digg user LtGenPanda's blog.
Digg stated that Digg employees were indeed behind the dummy accounts -- and that the accounts weren't built to game the system, but to test vulnerabilities that may lead to influencing how stories appear on the front page. Apparently these accounts made Digg staff learn a whole lot and prompted some changes to the code over the past few weeks, with more tweaks to follow.
Digg founder Kevin Rose himself took the time to stress that Digg has been doing such tests "since day one" and that Digg never received any money from any publisher partner for anything other than standard ad units.