According to the hacker, there isn’t a specific reason for which the sites have been defaced. They simply did it to demonstrate the lack of security measures.
One of the hackers has revealed that the team is planning to declare cyberwar on a country. They couldn’t reveal the country’s name since the final decision hasn’t been taken yet.
They also plan on releasing their own hacking forum within the next 20 days. The complete list of affected websites can be found here - most of which are still defaced.