So apparently YouTube, according to a Pakistani telecommunications authority, carries content that is “deemed offensive to Islam.
So the ISP either purposefully or accidentally, added custom routing configurations to it’s routers to block YouTube. The unfortuate side-effect was that these BGP announcements were propagated to a large part of the world’s routers, taking YouTube offline for a good number of people.
This fellow has a pretty good summation/chronology of what happened:
Posted by Brian Blood in Routers and Firewalls, Soap Box