There is a whole blog on Cracks in Sidewalks, by someone with a deep and abiding fascination with cracks in sidewalks. They are caused by the gradual expansion of the netherworld. The netherworld people are upside down under there, a mirror to our own world. As more and more of them are born (when people in this world die), and they are all walking around on the underside of this world, it's obvious the surface has to stretch. Usually our existence is enough to keep things in balance, but in areas where a lot of people die and not enough new ones are born or move in, the weight doesn't even out.
In the picture is an example from our block.