Hikikomori is the Japanese term for "Those who isolate themselves". These are generally young men between the ages of 15-25 who withdraw from the rest of society. The word means "to withdraw" or "to close", and isolate.
There is not a single reason for these people to isolate themselves from the rest of the world. Many believe that it is because of the pressure to "be like other people, and if you aren't, you have a sense of loss, of shame. So you disappear." Since Japan is so trendy, and the society has a heavy bias towards "popular", and an "in-crowd", if you aren't part of that crowd, it is very frustrating to be apart from it, and even more frustrating to keep up with it.
Many Hikikomori are popular kids in school, who just run out of energy to stay in the competitive world of a pop culture that moves at the speed of light. These kids lose sight of their goals, and isolate themselves from a society that they no longer identify with, or care to be a part of.
Until recently, the Hikikomori were just seen as otaku, video gamers, and people who watched too much TV, or read too many books. They would surround themselves with the things they loved, and not venture far from it. The internet is now another aspect that has gained popularity with the Hikikomori, because it allows them to connect with people with similar interests, without having to worry about meeting them in the real world.
Earlier this year, there has been a wave of violent crime in Japan. One baseball team member came to school, and beat other team members with the bat, injuring many, he then went home and beat his mother to death. In another incident, a young man broke into his next door neighbors house, and walked room to room, stabbing the sleeping residents, killing 3. A 17 year old boy hijacked a bus with a knife, and killed passengers along the way on a 19+ hour journey. In another incident, a young man killed a sleeping passenger on a commuter train with a hammer, "to experience killing".
Now new light has focused on the Hikikomori. In a recent wave of teenage violence and crime, the blame has fallen on the Hikikomori. Just like the Trench-
coat mafia, and the "goth" kids in the United States took the blame for teenage violence, now the Hikikomori are getting it in Japan.
Hikikomori are just antisocial, the don't have the energy to live in the "appearances matter" world, or just don't care to live in it. They don't feel like using energy to "look good", or "fit in". They would rather use their efforts in other ways.
Now the Hikikomori must deal with the fact that their society is looking at them as psychopaths, and killers. Thats just more reason for them to stay indoors.