What causes the progression from a simple fold to an over-fold?

The progression from a simple fold to an over-fold typically occurs due to increased tectonic stress and the accumulation of additional sedimentary layers. As compressive forces act on the rock layers, they can cause the folds to become more pronounced. If the stress continues to exceed the strength of the rocks, it can lead to more complex deformation, resulting in over-folds where the layers are tightly compressed and may even invert. This process is influenced by factors such as rock type, temperature, and the rate of tectonic movement.