Just to clarify, it isn’t always just a fear of catching yourself on a bad hair day; sometimes eisotrophobia is the fear of being put into contact with the spiritual world through a mirror.
More than often, it pertains to idea that mirrors have supernatural properties. Like with most phobias, suffers realize that their fear is irrational, but I suppose that just makes it all the more scary.
Fear is often grounded in superstition and thus suffers may worry that breaking a mirror will bring bad luck or looking into one will transport them to a land of infinite twerking Miley Cyruses.
For many people, this fear occurs during certain parts of the day; for example, in the evening, when bloody Mary is surely lurking behind the delicate plane of glass.