I also received the reset email and my password for dropbox was a random string generated by lastpass so I'm fairly confident that it wasn't leaked elsewhere. I wouldn't be alarmed if you got the email.
FWIW, I use Dropbox with multiple accounts and did not receive a "reset your password" email.
It would be useful info to know what triggered their email.
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).