Seriously thinking of quitting this site for good (well, not really).
OK, normally we don't comment on moderation actions, but given the scope of what happened (and, really, the fact that you all figured it out anyway), we thought a brief statement is in order.
For quite some time we've been investigating a pattern of fraudulent activity involving multiple accounts. When we deleted several accounts which were engaged in vote fraud, it also wiped out the votes they cast. If you were the recipient of any of those votes, you'll see a reputation change reversing the effect of the votes, along with the message "user was removed".
Now for something completely general: I believe it's public information (somewhere on the site or Meta SE, but I forget where) that "fraudulent activity" includes various ways in which one person uses multiple accounts to do things they couldn't do with a single account. Things like
and so on. The point is that when you lose reputation due to the deletion of an account, it's usually reputation that was not fairly earned in the first place - not through any fault of your own, but because someone else cheated the system to give it to you. And if the activity has been going on for a while, the amounts of reputation involved can be pretty large.
I hope this is enough to satisfy people's concerns. We would like the community to return to the status quo of not discussing details of individual moderator actions, but at the same time we want everyone to be clear that we (mods, SE, the computers... hopefully) are not up to anything nefarious.
I am afraid that, as usual, we can't comment on moderation actions.
I will echo tpg2114's comment that users are only ever removed because
They explicitly request it.
As a consequence of breaking the terms of service
The Meta StackExchange faq on the matter is What does “user was removed” mean and why did my reputation change because of it?
HDE 226868 asks "Isn't there something in place to keep the votes of such high-voting users?". I believe that the deletion of high-rep, longtime members is handled very carefully on a case-by-case basis to avoid---well situations very much like we have here. As far as I can tell there is no faq about that on Meta StackExchange
On a personal note, I'd hate to see you go because you bring a considerable expertise on a subject I don't know nearly enough about. I can appreciate that sudden changes of that magnitude are a heck of a shock and break the immersive nature of the gamification, and I couldn't blame you if you decided to give the site a rest for a while. But I still hope that you'll be back some time.
Whoa, 1,741 votes removed! (How on Earth did they manage to upvote me the 1 rep point?)
I've noticed that over the last few weeks I've been getting lots of upvotes on old questions, and in her answer to Serial up- or down-voting reversed Gwen said she had been reading old questions and upvoting the ones she liked. I also note she's no longer a member, so that's an obvious contribution to the debit. If so I can't really complain as they were upvotes I hadn't really earned anyway.
From various comments I wonder if there has been a group of friends recently join the site and vote in similar ways. If so that might explain the mass defection (I should make it clear this is just mischievous speculation not inside knowledge).