It's apparently a folk etymology, though the ultimate source is unclear. The earliest recorded forms are things like pick pack and pick back. The OED says, "Perhaps a combination of pack n.1 and pick v.2 (i.e. ‘a pack pitched (on the back or shoulders)’, as suggested by the α forms), or perhaps of back n.1 and pick v.2 (i.e. ‘pitched on the back or shoulders’, as suggested by the β forms)."