The process of popcorn popping in a microwave oven is endothermic because:
During popcorn popping, the kernels absorb energy in the form of microwaves. The microwave radiation heats the water molecules present in the kernel, causing them to turn into steam. The pressure from the steam builds up inside the kernel until it reaches a point where it bursts, turning the kernel inside out and resulting in the familiar popcorn shape.
Since energy is absorbed from the microwave radiation to facilitate this process, the popcorn popping is considered an endothermic reaction. The endothermic nature of the process is why the popcorn bag and the popcorn itself become hot during microwaving.