The short answer is nobody really KNOWS who is attacking Sony or why specifically Sony is getting attacked so often, but their track record can give us a little more insight.
Back in 2010, Sony stated on their page that they'd welcome open source operations on their game machines. As such, the Linux people (Guys who make computers do things that aren't Windows or Macintosh based) got super excited. PS3 with new tech? and they're giving us the go-ahead? Awesome!
Then in the same year, Sony patched firmware disabling the Linux portion of their machine. A Class Action Lawsuit was filed, but dismissed,, stating the plaintiffs could not prove that they had a right to expect the OS feature beyond Sony's warranty period or continued access to the Playstation Network (PSN).
As you can imagine, the Linux people got maaaaaaaaaaaaaaaaaaaaad. Whether or not this lead to more direct attacks against Sony's infrastructure is uncertain, but it stands to reason that's the case. You have to remember, people who develop Linux and Open-Source software are really, SCARY smart. These are people who code, develop software, code more, and ENJOY it. And Sony basically gave them the middle finger, saying "No, you guys suck. You can't play with our toys."
To add to this, the PSN outage of 2011 was a Public Relations nightmare. Not only did Sony basically hide it from the consumer that their systems were compromised and credit cards / personal info was leaked, but when PSN finally DID go back up and people were trickling back in, their 'thanks for not hating us' promotion kinda felt cheap. "Oh good, I can have a month free of a service you want me to pay for." Well, then they threw in some older free games, but they all had sequels with them, so it felt more like an upsell than an actual apology.
There's a few other grievances with Sony's movie house that i'm not as familiar with. But the long and short of it is Sony's PR is kiiiiiiiiiinda terrible.
Hope that helped. yes, it's very opinionated, but again, there's not a real clear answer as to who's leading the hack attacks or why specifically Sony. Yes, some are gonna point to Anonymous but they... Well, they're easy to blame; Anyone can be Anonymous. (Not REALLY true; Anon has moving people but their hivemind opinions are pretty clear; free speech and public information. They've never been known to do finance or banking attacks before. But that's a whoooooooole 'nother ELI5.)
As an aside, I believe the Extra Credits crew did a good job explaining why handling the 2011 PSN outage did poorly, but this is mostly opinion based, so take with some salt.