They can happen even with normal clotting function. I suffered a large one on my butt a few years back, it had to be drained by a surgeon a couple times.

If the nature of the impact causes a large sheering force on the skin it can “deglove” or separate the skin from the underlying tissue. The separation creates a pocket which will fill with blood. With most blunt impacts no pocket forms which leaves little place for blood to accumulate.