Why did the plates began to slip against each other 30 million years ago forming San Andreas fault

The currents of the mantle pull the crust above it through drag. The plates, therefore, move on top of the mantle. At San Andreas fault, the North American Plate met the Pacific plate. However, due to the presence of the Farallon plate that was disappearing into the mantle in a subduction zone, the Pacific and North American plates movements changed in their configuration. The two plates now move past each other. The San Andreas fault is, therefore, a strike-slip fault.