What is called sanity is mostly consensual. Most people aren't all that sane, just socially well adjusted. You know - the sort of people who can comfortably fill authoritive roles, and function well in society as it's structured. When the social certitudes crumble the sane people unravel. Like this site for instance; the most naturally sane people like martis, goblin, brucerobertson, henrikc, and a small army of others were banished until only the socially sane remained.
