system wordpress/drupal confusion
I haven't experienced this myself, but -- skimming the code in index.php -- it appears to track a variable called civicrm_install_type. The variable defaults to drupal but can be set to wordpress by passing ?civicrm_install_type=wordpress. Once it is set, it's stored in the SESSION and reused for future page-views.

One way you could probably trigger this behavior is to install WordPress+CiviCRM, swap out the code base, and replace it with a Drupal+CiviCRM codebase (on the same server, using the same browser session).

If this were the situation, you could get around it by either:


Resetting the PHP session (eg deleting cookies or logging out).
Explicitly passing ?civicrm_install_type=drupal when visiting civicrm/install/index.php.