Dropbox seems to have emailed all their users requiring them to change their passwords.
The company said in the mailer:
Recently, passwords have been stolen from some Internet services. This is a problem because many people use the same password on multiple services, which is unsafe. As a precaution, we’ve reset your password and you can create a new one here.
Dropbox added in the message to me that no suspicious activities were performed on my account and this is just a precautionary measure.
It is not clear if this is related to a new security breach on their servers or they are still recovering from the previous hack.