content: They use a distributed file system (meaning that it's like having a lot of harddisks on a giant network). Sometimes those harddisks are bottlenecked serving up other videos. It's like a 2% chance that people randomly want the same videos on the same disk and that the needle can't read from both videos at the same time basically + all other nodes that have this video duplicated on it are busy. This is google's fault and it's so shitty how they blame ISPs.