Yeah it was abit unpredictable last night, I got d/c a few times while playing MS:A, it seemed to calm down for me after 9pm though. It was pretty poor the other night, I did think it was my wireless so I moved the PS to plug it into the router and I was still getting a d/c or not being able to connect at all.
The hackers are doing it just to tarnish Sonys service, if customers can't connect and Sony can't fix the problem quickly they may lose customers to other consoles. Some customers do pay for a service called PS Plus, so I'd imagine those people are pretty miffed to be paying for something that isn't working!!