Most humans live within a certain comfort zone of acceptable behavior. They have learned to mostly do what feels familiar and safe to do. They stay within their boundaries. They do what is already known and predictable.
Most people appear relatively sane.
Their lives have a certain system and consistency to them. They can explain their actions, they have recognizable patterns of how they do things, and their course of action is fairly predictable. It is common to have fears of anything that lies outside those norms. Anything that is different, inconsistent, unpredictable, unexplainable, and extraordinary is often considered dangerous and unsane. Particularly anything that is moving rapidly in forceful but totally unpredictable ways can easily be considered insane.