Cavitation burn occurs when water flows over a surface and due to the speed/shape or surface condition, creates a low pressure area literally causing the water to boil. It can be from a dinged up leading edge on an impeller but can also be from too much cup in the blades, too rough a surface, casting flaws, Too small an impeller cut etc. Think of a small stream with rocks in it, as the water flows around the rock it creates an eddy behind it, that's the areas that if going fast enough would lower the pressure to the point that the water will boil.
