Lack of experience of the visual cues required, combined with an unsympathetic mechanical input. Practicing low approach and go around at a suitable height is priceless when dealing with the "ground shy" student. Fly level at five feet and don't try to land. When happy, close throttle and don't try to land. Demonstrate to the student just how much elevator authority they really do have. At five hundred feet, brief "never ever put the control forward, just fly level" Learning not to put the aircraft on the runway is paramount to not nosing-in. The landing attitude is the same as the take-off attitude. Remind them of that when they line up. Above all, relax and fly. Don't intellectualise.