Well, I'm not a hacker, so by no means would I know. If I had to guess from being around computers, I would say they went in through a back door, or basically where able to hack in using someone esles log in information. Once in, they probably loaded a trojen, or some other worm, etc and from there they turned the PSN into a slave system (meaing able to follow prompt commands). Just a total guess though.
People are very sophisticated in their ability to do stuff like this. Once they target something, they are pretty good at getting in. It is basically a tug of war match now. The hackers do something, Sony counteracts, and vise versa, until Sony eventually is able to block them out. They may also have it down to protect end users from their data being infiltrated and destroyed.