Hikikomori is not only a Japanese phenomenon. I was Hikiki for a year after dropping out of college.
I lived with my parents playing WoW all night and sleeping all day. I had very few social interactions.
What dragged me into that lifestyle was the complete destruction of my ego. I was simply unprepared to live as an adult. It's not much of a surprise I couldn't cope. In high-school I was coddled for being smart and never developed a work ethic. College hit me like a sack of bricks.
I'm sure there are myriad reasons why people choose to withdraw from society; my case is merely a personal anecdote. Nonetheless I think my case is not a rare exception.
Ultimately what pulled me out of that malaise was successful employment in a software company. It was only after I recovered a sense of self worth that I could finally step out of my cave.