Log WooCommerce order status changes with debug backtrace to determine what caused it.

log-order-status-change.php

<?php

/**
 * When the status is set to processing, record what caused/initiated the change.
 *
 * Need in `wp-config.php`:
 * define('WP_DEBUG', true);
 * define('WP_DEBUG_LOG', true);
 *
 * @hooked woocommerce_order_status_changed
 *
 * @see WC_Order::status_transition()
 *
 * @param int    $order_id
 * @param string $status_from Un-prefixed WooCommerce order status.
 * @param string $status_to
 */
function log_what_set_status_to_processing( $order_id, $status_from, $status_to ) {

	if ( 'processing' === $status_to ) {

		$url = admin_url("post.php?post={$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', 'log_what_set_status_to_processing', 10, 3 );