Meanwhile, the crowd has also found a not so irrational explanation for the sudden block. Marc van der Chijs, of competitor Tudou:
The reason? Probably not directly the ongoing 17th National Congress in Beijing, then they would have blocked the site last week already (unless someone right now uploaded some video's that would upset the government). I suspect the real reason might be that YouTube just launched a Chinese version, which would make the site much more accessible for Chinese users. Not a very smart idea to do that in the middle of the National Congress, and I am surprised nobody at mother company Google's China offices rang an alarm bell about this before the launch. A typical example of the mistakes foreign companies make while trying to do business in China.
I had missed that launch, but it is very very plausible explanation we get here.