You're on the right track but you're not tying the pieces together quite right.

Heaterhate gave a good explanation of why the supporting strata sloughed downwards; the side of a hill was terraced with fill to make it level. The fact that it was filled and compacted means that it likely didn't bear as much weight, they possibly didn't excavate before they filled on top of the natural slope creating a shear plane and combined with the fact that there was nothing on the down sloped side of the hill allowed a large amount of material underneath to displace. Had they made the terrace another 10 foot wider there may have been enough material (a bench) there to prevent such a large amount of displacement.

sssheild is correct as to why the split was so clean (the opposite travel lanes were paved at different times so there was a cold joint in the middle). Basically the joint was formed after the asphalt mix cooled, so there is no intermingling of the material and minimal adhesion making it was the easiest separation point for the crack to follow and any crack will always follow the weakest point unless there are extreme lateral stresses (expansion and contraction of a thin concrete slab with large footers or walls within or around it is a good example) and then the cracks often follow the stress vectors. When the soil below it moved the elasticity of the asphalt allowed it to deform to fit the contour of the material underneath it while providing a flexible cover to keep the now loose soil at the top somewhat together and it moved as a unit.