According to Fox:
Fear of being committed may have caused Connecticut gunman to snap

“From what I’ve been told, Adam was aware of her petitioning the court for conservatorship and (her) plans to have him committed,” Flashman told FoxNews.com. “Adam was apparently very upset about this. He thought she just wanted to send him away. From what I understand, he was really, really angry. I think this could have been it, what set him off.”

A senior law enforcement official involved in the investigation confirmed that Lanza’s anger at his mother over plans for “his future mental health treatment” is being looked at as a possible motive for the deadly shooting.

Flashman was told Nancy Lanza had begun filing paperwork to get conservatorship over her troubled son, but that could not be confirmed because a court official told FoxNews.com such records are sealed. The move would have been necessary for her to gain the legal right to commit an adult to a hospital or psychiatric facility against his will. A competency hearing had not yet been held