I'm curious who all received this email? Was it sent to the entire user base? If not, what selection criteria did they use?

Everyone I've talked to seems to have received the "reset your password" email. I'm quite curious because I'm certain (up until now) that the password I used for Dropbox was both (a) not commonly used and (b) had been changed recently and (c) not leaked anywhere else (to the best of my knowledge).