I think it's several factors:
- No WCS circuit or EWC this year. EWC teams dropped the few SC2 Korean players they had. (Classic was dropped by Virtus.pro in September.)
- There was a terrible patch during 3-4 months until they reverted, at the start of October, to the previous patch, which was not that great.
- The SC2 ladder is full of hackers sniping the top GM who can win or tie when they decide. Blizzard banned the hackers when publishing the patch, but the hackers came back at the top after ~48 hours.