System changes order status back to processing

Hello!

I found some strange things on my WC-store. When the customer make an order system change status to “pending” with this hook:

add_action( 'woocommerce_thankyou', 'custom_woocommerce_auto_complete_order' );
function custom_woocommerce_auto_complete_order( $order_id ) {
    global $woocommerce;
     if ( !$order_id )
        return;
    $order = new WC_Order( $order_id );
    $order->update_status( 'pending' );
}

Then absolutely randomly system change one or more orders status back to “processing”. Usually that happens the next day.

Any ideas?

https://wordpress.org/plugins/woocommerce/