Dropbox has a very handy feature for NSA/GCHQ.
This is very likely a change to Dropbox that has been subpoenaed on them, along with a gagging order, by the incoming Trump Presidency.
Given the timing. It's likely Powers that be, said they want additional access to all deleted files on those Dropbox accounts too, not just active files as an extension to the current access granted via a NSA portal (whether by Court order or not, the detail is unimportant in this context).
During the process to put this measure in place, in retrieving past deleted data to meet this order, they accidently exposed the new measure to users too. Dropbox has always been a target for NSA/GCHQ because of its handy security flaw/"feature" (see below). Dropbox stores files unencrypted (unless you pre-encrypt those files), accessible by a few key staff.
Dropbox has always had a security flaw, in that you can present any Dropbox account - a file, and it will instantly appear in the account if that file already exists somewhere in the Dropbox cloud, with no need to upload it.
So you can throw 'known files' (say, known to contain illegal content/Intellectual IP) at it and if they are "already up there" (in Dropbox's Cloud) the file will instantly appear on the dropbox account without uploading it, Dropbox effectively "give you that file", because they can see you already have it. If the file starts to upload (i.e. Dropbox doesn't have it), it can be cancelled, so doing this is cheap, effective and not a security threat "as such".
If Dropbox worked with Security Services, you could have a method to do AI based part matches, with a percentage match, but out the box, Dropbox is good for matching exact files.
NSA/GCHQ then go knocking on Dropbox's virtual door (which in reality is via a NSA portal), to get all the accounts that specific file is been stored. i.e. a very handy "fishing tool", just on the right side of legal, because its using the flaws/"features" of Dropbox.