To answer your question more precisely, when the rifle recoils and moves backwards, the round(s) in the magazine don't move with the gun instantly. When playing 'catch-up' during the recoil process, the tips of the bullet are smacked against the front of the magazine during recoil, similar to what would happen if you held one of the rounds in your hand, then smacked it into a table point first. Because there isn't enough neck tension to prevent bullet setback, the bullet slides deeper into the case. This isn't a normal thing for correctly produced ammo, but it will occur more often in magnum calibers and light weight rifles.
