Add this snippet to your functions.php file to randomize your related products. Edit the pool size for a larger set of random products.

randomize_woocommerce_related_products.php

  add_filter('woocommerce_related_products_args', 'my_related');
  function my_related($args) {
    global $product;
    $pool_size = 10;
    $related = $product->get_related($pool_size);
    if ( sizeof( $related ) == 0 ) return $args;
    $args["post__in"] = $related;
    return $args;
  } // end function