How does overtillage harm soil?
I am no expert, but over tillage would be similar to a process where you start to build a game like Jenga, but keep knocking it over before it's finished.
Soil, like many things, needs time to rejuvenate and replenish its rich ingredients. Whole ecosystems live beneath the ground which can help the soil for plants you are going to grow. If you keep tilling the land, it will never have time to settle. That's the just of the problem, but again I am no expert.