They're probably upgrading their servers, changing some features around, or just reprogramming some stuff. They have to do that live because a lot of people would be furious if YouTube went completely down for even a few hours. This way, only a few core features go down, while the rest of the site functions as normally expected.
10,000 Posts - 10/13/2011
Become Minion - Not Done
Become Donator - 07/29/2011
1000 Thanks - Done - 07/25/2011
2000 Thanks - Not Done
Thanks Archangel for my 1,000 Thanks.
Thanks Keroaplt for my Signature.