Usual totalitarian bashing aside, this may actually be the real reason:
Back ground info: Spring Festival is coming (this year it's Feb. 10th), which calls for all people to go home to unite with their family. And this makes the train tickets very difficult to obtain and the beginning and ending the holidays. The railway ministry in China has build an online train-ticket buying/reserving system (12306.cn) a couple years back and it is now well known when new tickets would be available online, and they sell out within minutes.
As a result, it brought down GitHub a couple weeks ago (when the grabbing of this years tickets begin), and the ticket sellout windows went from minutes to seconds.
The railway ministry is pissed and claim that this practice is "illegal" or "immoral" and should be banned. Blocking Github could just be the attempt at blocking that "ticket assistant" plugin: No Github, no plugin.