I believe the blocking is directly related to an automated train ticket polling plugin that had brought down Github just days ago. Due to upcoming Chinese New Year, newly released train tickets are sold within minutes. That plugin introduces huge traffic to already crumbling ticket vending site, and it has obviously made railroad bureau angry. Blocking Github makes that plugin immediately malfunction because it makes reference to javascript files hosted on Github pages. The bureau has also paid a visit to the plugin's author, for the purpose of intimidation perhaps.
ccp0202 is absolutely correct. seems that no american media or organization know what the heck this is for yet. but you can check out some chinese sources.
