From my understanding the biggest deciding point is portability. C++ is very portable and be easily ported to different operating systems. C# is designed for Windows using the .net library. You can acheive some portability with it by using a third library, like Mono. Much of the syntax is very similar between the 2.
My guess is that the underlying code that handles the "heavy lifting" is C++, and the UI stuff is C#. Why C# when they are also using Qt (multi-platform GUI API's) is beyond me. Totally unnecessary IMO.