Determine what changed the order status in WooCommerce

order-status-change.php

<?php

/**
 * When the status is set to pending from completed, record
 * what caused/initiated the change.
 *
 * @hooked woocommerce_order_status_changed
 * 
 * @see https://www.reddit.com/r/woocommerce/comments/mi1f85/status_on_old_orders_change_from_completed_to/
 * @see WC_Order::status_transition()
 *
 * @param int    $order_id
 * @param string $status_from
 * @param string $status_to
 */
function check_what_set_status_to_pending( $order_id, $status_from, $status_to ) {

  if ( 'pending' === $status_to && 'completed' === $status_from ) {

    $url = admin_url( "post.php?post={$order_id}&action=edit" );

    error_log("\n\n\nOrder {$order_id} status changed from {$status_from} to {$status_to}.\n{$url}\n\n");
    error_log((new Exception())->getTraceAsString());
    error_log("\n\n");
  }
}
add_action( 'woocommerce_order_status_changed', 'check_what_set_status_to_pending', 10, 3 );