What led to that conclusion? Genuinely curious.

He runs a voting ring.

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.

I've seen this, I tend to attribute it to a different part of the world waking up.

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.