I really dont think nations are DDoSing TPB, maybe some rouge parts of the media industry...
And I think its not kids DDoSing pools either, I can hardly imagine some script kiddies are in control of botnets big enough to do that... I think its a couple of guys that really know what they're doing (of course DDOS is bitchy and childish, but using a botnet to do mining is hardly something you can do by after reading "Hackers Black Book"...)
I think, there is a lot of money involved in this and of course a lot of rage after beeing banned...