I don't think everyone realizes how large Anonymous is.
It isn't a group, it's a network with cell's spanning 10's of thousands of people across the globe. (which makes it virtually impossible for any gov't agency to take it down).
If 20,000 people did a DDoS...hitting servers across the globe..it's deff. possible.
However, I'd imagine that it would be a combination of DDoS attacks, Infrastructure hacks, and possible having someone (or multiple people) on the inside.
What's more interesting is that they choose their attacks based upon a democratic voting system.