I think the most common causes are excessive scratching or hitting the ear on a hard surface during vigorous shaking. In this case I don't believe it was from scratching but other than that I have no idea.
For some reason the vet is reluctant to drain the hematoma. I think the swelling is excessive but they went to vet school and I didn't so I am trusting them.
They stitch the ear up to produce scarring to add shape to the ear lost from cartilage damage.