My sand is notorious for doing that but this is the worst example I have ever seen. What happens is silica sand goes through a very rapid expansion phase when molten metal flows over it causing the mold surface to buckle. Most of the time it happens when molten metal flows over one spot more than other areas. I rammed the cope too hard and it does not allow the sand to expand so the mold surface distorts. To fix this problem wood saw dust is mixed in the sand and burns out before the sand can expand.