Does dropbox have many Chinese customers ? Has it been identified as a 'circumvention device'?
As to it being identified as a 'circumvention device', there are many reasons why it could have been blocked...some automated methods and some that require a person. It could be as simple as someone published a file with certain keywords in it and that file link got posted in a few places...then bam...automatic block. We don't know the rules to the system. And there are actually several systems, run by different government departments...with support from many more non-government employees of web sites.