Here is what happened: that account got mistakenly upvoted by me - hence the reputation increase. He then proceeded to upvote a bunch of his other accounts and downvote others.
Again, this was a mistake on my part so I undid the upvote. Other admins downvoted this account as well. Because of how the reputation protocol works this is all that was needed to cancel out all of his other actions.
The reputation system is not perfect and admins will make mistakes from time to time. However the damage that can be done is limited and attacks can be easily detected and undone by admins.
Notes on the downvote attack:
We do see some users that earned reputation via legitimate ways downvoting quality articles in order to increase their own payouts on other posts. This is not ok. If we see people doing this consistently they will get downvoted. We are working on providing better visibility on article downvotes as well as relationships between different user accounts so everyone can track anomalous, manipulative behavior.
Thank you to everyone for flagging this. However [@ukaw13](/user/profile/ukaw13) and others, it is not OK to spam comments / feeds. We get notifications, one post or comment is enough.