Uneven concrete is caused by voids opening up below concrete slabs. These holes are usually caused by different types and densities of dirt being mixed during the construction process, then being placed under the pressure of concrete slabs, the ground starts to settle. These mixtures of dirt are also susceptible to erosion. Both rain and snow can seep below the concrete to wash away the lightly packed dirt, but this isn’t the most common reason. Usually, if the concrete is sinking, there is an underground water pipe that has broken and the water has washed away the looser dirt from below.