C++/C# repositories have more or less doubled since Jan16 (from 10000ish to 20000ish) and so have Java ones (75000ish to 150000ish).

So, there is not really an uptick in terms of percentage, the rise is more apparent in Java (and Javascript) case since they were more popular in the first place.

To see the relative shift in popularity, you would have to normalize the data against the total repos, I think.