"Fight or flight"
It's an involuntary response. Other wise knowns as "hyperarousal", it's how your body reacts to situations that cloud your thoughts with complete uncertainty. A harmful event, such as an attack, or a threat on ones survival.
Fight or flight, have you thought about it? How would you respond if the very ground you're standing on was torn from beneath you?
In Pipers situation, she ran. She ran so far away and started over completely. Leaving her past behind her, she stumbled into the seemingly friendly town of Charming, California.