My reputation dropped by 70 points taking me below the magic 3000 mark (I was kind of excited to be able to cast open/close votes). There doesn't seem to be any rhyme or reason for the drop (I had no recent downvotes, for example). However, I'm sure there is a logical explanation that I'm not aware of. What is the likely cause in this case?
Reputation can change for other reasons than up and downvotes. If a user deletes a question, for instance, all of the associated reputation earned with that question would be deleted. So most likely, a user deleted either a question (or perhaps even an entire profile). That would explain a sudden reputation drop.