The decision was reportedly made just recently, because NVIDIA wants to synchronize the naming of its desktop and laptop platforms. There's already a 800M series for laptops and with the 900M mobile GPUs coming up soon NVIDIA didn't want to add to the confusion so both the upcoming desktop and laptop GPUs will use the 900 series number scheme.
NVIDIA is planning a big press event around September 9-10, this will be a paper launch to announce the second-generation Maxwell architecture. Earlier this year NVIDIA introduced the GTX 750 and GTX 750 Ti, those cards were based on the first-generation Maxwell. The actual NDA lift date for NVIDIA's new cards is September 19th, on that date you can expect to read lots of reviews of the GeForce GTX 980 and GeForce GTX 970. A GeForce GTX 960 model will follow in October, a well as the GM204-based GeForce 900M series.