--root required for all drush 8 commands and I don't know why

Question

Does not work:

root@deftig-web001:/var/www/1.websiteserver.net/platform# drush status --uri=dev052
 PHP executable         :  /usr/bin/php
 PHP configuration      :  /etc/php5/cli/php.ini
 PHP OS                 :  Linux
 Drush script           :  /var/www/vendor/drush/drush/drush.php
 Drush version          :  8.1.11
 Drush temp directory   :  /tmp
 Drush configuration    :
 Drush alias files      :

Works:

root@deftig-web001:/var/www/1.websiteserver.net/platform# drush status --uri=dev052 --root=/var/www/1.websiteserver.net/platform
 Drupal version                  :  7.59
 Site URI                        :  dev052
 Database driver                 :  mysql
 Database hostname               :  127.0.0.1
 Database port                   :  3306
 Database username               :  root
 Database name                   :  dev_dev052
 Database                        :  Connected
 Drupal bootstrap                :  Successful
 Drupal user                     :
 Default theme                   :  t_heftig
 Administration theme            :  t_heftig_admin
 PHP executable                  :  /usr/bin/php
 PHP configuration               :  /etc/php5/cli/php.ini
 PHP OS                          :  Linux
 Drush script                    :  /var/www/vendor/drush/drush/drush.php
 Drush version                   :  8.1.11
 Drush temp directory            :  /tmp
 Drush configuration             :
 Drush alias files               :
 Install profile                 :  heftig
 Drupal root                     :  /var/www/1.websiteserver.net/platform
 Drupal Settings File            :  sites/dev052/settings.php
 Site path                       :  sites/dev052
 File directory path             :  sites/standaard/files
 Private file directory path     :  sites/standaard/files/private
 Temporary file directory path   :  /tmp

But since I'm executing from the Drupal root, I don't see why --root is necessary. What am I doing wrong?

Drupal: 7.59 Drush: 8.1.11