Did you know that there are two shapes of popcorn, there is the butterfly and the mushroom. In both types, what happens when the heated popcorn, is that it’s that popping pressure that causes a failure in the outer skin of the kernel. Even though the kernel skin is quite tough, the hole expands with the heat and the skin stays mostly together. The starches within the kernel are blown out through the hole, which causes the popcorn to invert, so that the skin is now on the inside, and then as it cools it cracks in several places.
When the expansion occurs evenly, you get ma mushroom shape popcorn, which is roughly spherical, with the outer surface developing fissures to resemble a mushroom.
Some varieties of corn don’t expand in an even fashion. The corn kernels actually have several lobes. When these lobes expand, they expand away from each other, which results in a butterfly shaped popcorn.