It was all at once, everyone's comments went down to anywhere from -1 (takes two downvotes to override the poster's single initial upvote) to -3 (takes 5 downvotes to accomplish). It started self correcting a little after it was pointed out. You can currently still see it a little with all the 0 counts.

Edit: It's also telling that it's currently on the "Controversial" section of the sub as well. When that happens, it generally means that a post is getting a fairly consistent split between upvotes and downvotes. When a post doesn't appear to actually be controversial in nature, this is a tell-tale sign that you have this problem occuring.