Seems reasonable to guess. I guess if Trump wanted to defuse some of this for whatever reason, he could leak returns that are past their audit date - especially ones that show huge tax payments. But these could also have come from the cleaning crew rummaging through the files after hours - either his offices or that of his accountants? Ever see the film Wall Street?
If important files like this are in getting distance of a cleaning crew, you have really poor security practices.
Obviously, it's still wrong to take them, but secure documents should be secure.