No rule prohibited mass downvoting.

No rule prohibits burning down the servers that LW runs on. It's just that certain standards of behaviour are expected, here as anywhere else, and when rules are made, it is in order to clarify things where that is thought necessary.

Mass downvoting by one individual against another has emerged as a questionable phenomenon. Proof of its questionability: it is being questioned. Also, no-one has owned up to doing it, no-one has defended it, and if the motivation were concern for the good of LessWrong, the targets seem oddly chosen for that.

BTW, I was once the target of a bulk downvote. I had around 400 karma at the time and dropped 40. I thought, hahahahahahahahahaha! A dog barked; the caravan moved on. But the current practitioners appear to be operating on a larger scale.

This site, moreover, implies that voting is anonymous. Trike arrogated the right to determine voter identity: it invaded the user's privacy!

Voting is anonymous, in that your votes are not published to anyone else. That there is a database recording every vote that everyone has ever cast is obvious. How does the site ensure you only get one vote on each comment? How does it show you your own votes? Because it knows.

That such a database, given that it exists, will be examined in sufficiently egregious cases, is also obvious. However, I have not yet seen any instance of personal voting information being made public.