This can be caused by moisture penetrating the surface, later it freezes and expands. This will, in most cases, cause the surface of the concrete to "pop" and crack. I could also be due to the mix that was used being to wet or dry. Another cause yet could be using salt or "ice melt" on concrete that has not been sealed. When this happens it is very costly and next to impossible to repair with anything that will last long term. This can be prevented by properly sealing the surface with a product like SealantPro. SealantPro penetrates the surface up to 1/4" as opposed to most box store products you buy, which are typically "topical sealants" that coat the surface and will have to be reapplied annually.