If we knew what caused it, it wouldn't be IDIOPATHIC! If and when a cause is found, it will have a (probably mechanistic) name, and will no longer be called idiopathic.
The Ebright Road benchmark (NGS disk EBRIGHT AZI) was found to be 447.85 feet above sea level. Many people consider Centreville, Delaware to be the highest point in the state; however, a benchmark at Centreville has an elevation of only 445.58 feet making the Ebright Azimuth disk more than two feet higher.
The highest point is above 30 ft elevation (inside the 30 ft contour line). The second high point is above 20 ft in elevation, but does not reach 30 ft. These high points are at the ends of a ridge that runs the length of the island where elevations are above 10 ft.