As part of the Vault Experiment, experimental psychoactive drugs were released into the air filtration system of Vault 106 ten days after the door was sealed. The drugs affected some, but not all residents. This is evidenced by the note To Anyone Who Gets This, which was apparently written by someone who was not affected. It seems as though the drugs sparked violence in those they affected, resulting in the "insane" killing everybody else in the vault, leaving only a few survivors.
The insane survivors present seem to be unlucky wastelanders and raiders who wandered into the Vault (see note below), and were overpowered by the still-potent drugs into thinking they were dwellers.
The note entitled "To Anyone Who Gets This" is found behind a locked door inside the vault along with multiple skeletons, further evidencing that the insane 'survivors' are newcomers. And, along with all terminal entries, is referring to incidents that occurred over 200 years prior to the Lone Wanderer entering.