They sent both me and my wife an email a couple of days ago regarding this, and have a Help Center page[0] for it:
Hi <first name>,
We’re reaching out to let you know that if you haven’t
updated your Dropbox password since mid-2012, you’ll be
prompted to update it the next time you sign in. This is
purely a preventative measure, and we’re sorry for the
inconvenience.
To learn more about why we’re taking this precaution,
please visit this page on our Help Center. If you have
any questions, feel free to contact us at
password-reset-help%dropbox.com.
Thanks,
The Dropbox Team
