What invokes Bootstrap overrides.css?
The default Drupal Bootstrap 3 subtheme has all the required LESS files, which includes the overrides.less file. This file, and all others, is rendered into the style.css file by the LESS processor HOWEVER this can change if your processor is wrongly configured, so make that the first thing to check. Then check that your theme .info file is only referencing the css/style.css file and not any other css files.

https://www.drupal.org/node/2052931