Why are the potential charges if I am being accused of touching a girl of legal age while she was sleeping?

A charge of rape to begin with.

In Michigan this could be CSC IV? anyway which carries up to two years in prison.

Touching a person that is sleeping in a sexual manner is a violation of California Penal Code Section 243.5 Sexual Battery (c) Any person who touches an intimate part of another person for the purpose of sexual arousal, sexual gratification, or sexual abuse, and the victim is at the time unconscious of the nature of the act because the perpetrator fraudulently represented that the touching served a professional purpose, is guilty of sexual battery. A violation of this subdivision is punishable by imprisonment in a county jail for not more than one year, and by a fine not exceeding two thousand dollars ($2,000); or by imprisonment in the state prison for two, three, or four years, and by a fine not exceeding ten thousand dollars ($10,000).

Sexual battery, resulting in lifetime registration as a sex offender. Hire a lawyer before you do something you will regret in court.