What could cause or explain hiatal hernia in thin 28yr old female (never pregnant)? is this common in young non obese women?

Yes ,see below: It is a protrusion or herniation of the upper part of the stomach into the thorax through a weakness in the diaphragmatic wall. It can be due to high intra abdominal pressure due various causes, orheredity, smoking, stress, or diaphragmatic weakness!

Congenital: Hernias are unrelated to obesity and yes i see frequently young women with this problem but the incidence is higher in older patients.