A lot is poor trimming so pilots come in nose heavy, do not flare properly and as such arrive flat or on the nose wheel.
Good point Pace. I catch myself coming in without enough back trim from time to time and while I always land mains first on aircraft that have very heavy elevators anyway, like the Seneca, the lack of trim causes a tendency to de-rotate too quickly and bring the nosewheel down very firmly onto the ground rather than gently lowering it.