If you face an unexpected 301 or 302 redirect in Magento 2 and you don't know why it happens or what code causes it, you can easily find this out by temporarily editing the following files:
/vendor/magento/framework/HTTP/PhpEnvironment/Response.php /var/www/html/m2_35ee/vendor/magento/framework/Controller/Result/Redirect.php
Open Response.php and add the following line to the beginning of the setRedirect function:
var_dump($url); \Magento\Framework\Debug::backtrace(false, true, false); exit();
Example:
public function setRedirect($url, $code = 302)
{
var_dump($url); \Magento\Framework\Debug::backtrace(false, true, false); exit();
$this->setHeader('Location', $url, true)
->setHttpResponseCode($code);