I independently came to that conclusion after noticing that my comments on some controversial subjects will slowly acquire upvotes, then nose dive 2 or 4 votes very quickly. At first I thought that some mods might have double-value votes (Hi Cowboy Neal!) but then I realized it's sometimes 4 successive downvotes, not only 2.
I've frequently encountered this same thing. Often the downvotes come a day later, but quickly.
Same, but there are other explanations than voting rings
I assume it's just the mods, not voting rings. It's usually comments that run counter to the HN-approved ideology.
I've seen this, I tend to attribute it to a different part of the world waking up.