I fired our janitors and two weeks later the place was a mess. The janitors did it!
I fired the police and two weeks later crime rates were sky high. The police are the culprits!
It all depends on what Sony did to keep security up after it fired the workers. If they didn't replace them with at least temporary contractors or IT people from other departments, then they intentionally left their guard down. Strike when the guard is down. Just because the events occurred near each other is circumstantial. If I was working on penetrating Sony's systems, I'd increase my attempts if the security department just got laid off too.