Ulterior, IMHO. 
No conspiracy, he probably just didn't want to criticize the company he worked for while they were still signing his checks. The issues he's raising are true but nothing new, pretty bad form to shit on the company that made you rich so publically, but he can do whatever he wants.
Because he was sick and tired of Zuffa and the way they were treating him after all he'd done for them and he really didn't need to take that shit anymore.
-Would be my best guess-
When you work for someone on contract if you talk shit about them you will likely get fired. Dana has no problems talking shit about any of his fighters in the public so maybe GSP is just returning the favor.
Or you can think the other way..how gsp helped the ufc to grow and being one of the biggest ppv draw 
Seems to me that george really loves this sport and wants to keep it as competitively pure as possible. Even if it destroys his relationships or gets him hate he wants mma to be as clean as possible. Great to see someone pushing for more testing.