At the time, we were not able to confirm the defacement. No one was reported to have seen the hack. Even the Zone-h mirror didn't confirm the defacement, displayed a message "The mirror is onhold and has not been verified yet".
So, we didn't have strong proof to report the hack. Today, Rovio, creator of angry birds, confirmed that the defacement was there for few minutes and corrected immediately. Now, the Zone-h record also confirmed it.
Antti Tikkanen, Director of Security Response at F-Secure Labs, said in twitter that the attack is actually 'DNS Hijack attack'. He mentioned that the website itself not touched by the hacker; hacker managed to modify the DNS records.
He also said that the angrybirds website pointed to some IP address(18.104.22.168) assoicated with Lithuania for at least one hour. The same IP address shown in the Zone-h record(https://www.zone-h.org/mirror/id/21666969).
The hack comes after the angry birds application is said to be used by NSA and GCHQ to spy on people.