I've been the target of serial down voting before, so I'm familiar with the auto-script that will undo it if triggered, but the exact trigger is not publicly known. I'm also familiar with the secrecy of votes in general and that only certain developers can view them under certain circumstances. This seems like a certain circumstance that warrants a look to verify whether there's been a misuse of the SE system.
Here's a snapshot of my current reputation changes. You can see that I down voted on something, then 90 minutes later I received four down votes in a row.