That's problem #1. There should have been a joint in those 10x20 blocks. The driveway is mostly likely 4" thick. A single block of concrete should not be bigger than 10x10 @4" thickness. That's concrete 101 shit there. The concrete crew that did that are simply not very good. All the other points about subgrade, rebar, mesh, fiber-mesh, 3000 psi, etc....could be possibilites, but just by the picture, it is clearly the lack of a center joint.