It's the same cause of obesity as anyone: lack of activity and increase in calories consumed. It's very common to eat several times a day at work, as people will keep feeding you for some reason. The odd thing is Koreans will tell you that you're fat, but then if you try to eat light they'll ask what the fuck you're doing, if you're sick, etc., and that even if you need to lose weight you still have to eat plenty. One of my friends encountered "You need to eat 3000 calories a day" more than once.