The HDD has a platter (or platters) and an armature, kind of like an old record player. Even if the armature is retracted from the platter, it can be vibrated out of whack. And the platters, even if they're equipped with brakes, have mass that can be stressed by movements and shocks. WHile many advances have been made in their robustness, no HDD will be as impervious to jostling as a SDD.