// Set default orderby query to "rand" for shop archive pages
add_action( 'pre_get_posts', 'shop_default_orderby_rand' );
function shop_default_orderby_rand( $query ) {
    if ( is_shop() && ( ! isset($_GET['orderby']) || 'menu_order' === $_GET['orderby'] ) ) {
        $query->set( 'orderby', 'rand' );
    }
}