We found a voting ring with activities across several sites. Some users who contributed nothing but votes on Islam.SE were removed. These accounts built up reputation on other sites via fraudulent voting and used the network association bonus to vote on this site. Several other sites were affected by these deletions, but because of the relative size of voting, this site was hit the hardest.
Other votes were invalidated because of clear signs they were fraudulent. I won't go into details, but these votes were detected by automated processes we use to audit suspicious voting patterns. The voting pattern seems to have been designed to avoid detection by one of the oldest algorithms.
I should note that these accounts had been suspended and warned in the past about this issue. We gave them the benefit of the doubt at that time, since we try to assume good intentions. Unfortunately, the owner(s) of those accounts did not heed those warnings. If we'd been more aggressive back then, the pain of removing that many votes from the site would have been lessened.