"People who consider themselves hikikomori exhibit a wide range of symptoms, including depressive, autistic and obsessive-compulsive tendencies. A minority appear addicted to the Internet " That's strange, I always thought Internet was the main cause of this problem. I've imagined hikikomori as those in S. Korea or Taiwan who die of exhaustion while playing online few days straight.
They do often spend a lot of time on the internet, but it's not a cause, it's a related fixation (when you're trapped -- by what's probably some combination of your own preference and perceived outside forces -- in a room, the internet provides a wealth of things to do). It's a social malaise that might as well be an extension of the NEET phenomenon.