They did it for the lulz. PSN and Live attacked.

The short of it is that the self proclaimed cyber-terrorist group, Lizard Squad brought down both networks “for the laughs”. Then decided they’d recast their reasoning as an attempt to expose the vulnerability of both networks according to an interview with them.
The long of it. They felt Christmas would be best as it would anger the most people.  So whether the group is either just a bunch of criminals or are a necessary black hat to expose corporate laziness is open to interpretation.
What is not open to interpretation is that they are the grinch that stole Christmas. Who saved Christmas? Kim Dotcom did. He offered the group lifetime vouchers to his encrypted upload service, Mega.  (Yes, it’s the same guy some people call a “pirate” and the FBI is still very much wanting to arrest.) Sony and Microsoft were not able stop the attacks but thanks to Kim Dotcom, who wanted to play on console, folks can now get back to blasting each other to bits.
To hear the complete interview head over to WinBeta