Why is a National Hunt Flat Race called a 'Bumper'?

NH Flat races are called 'bumpers' because they were once for new jockeys to learn their trade and for not seasoned old sweats. You will nowadays see the likes of McCoy, Walsh et al in these races whereas once upon a time you wouldn't.

The name 'bumper' came about because these new and inexperienced jockeys were apt to bump up and down in the saddles rather than stay rock solid.