It was a fall line trail and eroding/widening very rapidly. Trails like that tend to erode since water picks up speed and washes the dirt away. That leads to more loose rocks and roots in the middle of the trail. As the middle of the trail is eroding and becoming more challenging to climb, people climbing will start riding the edge of the trail, widening the area of erosion in the process. After a few years of that you are left with an unsightly gully and a trail useable by a few remaining experts or no one at all. People with more sustainable trail building knowledge can give you a more thorough answer.