Cracks will usually form due to the oven walls being too thin or the material was not completely dry when performing the first firing. When heat builds in the oven and water remains in the walls, the water will turn to steam and attempt to escape the walls quickly and thus create cracks. These cracks are unlikely to cause structural issues and will likely only be unaesthetically pleasing. Simply smooth over the cracks with similar material the walls were made of (Eg. Clay or mortar). It is also possible that water will re-enter the oven walls via rain if the oven is not protected from the elements. When water seeps into the walls this water will again try to escape with the subsequent firing. For this reason you should either ensure the oven dries completely before use or build a cover oven your oven to prevent it becoming wet.