They've been having DDoS attacks since September last year, so it seems unlikely that it's caused by a recent event. I'm surprised they haven't done anything about it before now.
The attack in September occurred just as a large number of Koreans suddenly moved to Telegram. The mass exodus was triggered by a surveillance scandal where a major Korean competitor was found to be handing over a large amount of user data to the government.
In recent days, there's been another exodus of Koreans from domestic IM services due to the revelation that the Korean army has been a customer of Hacking Team, the Italian spyware vendor who got hacked last week.
