Why would my CiviCRM CSS file not load correctly?
I would guess it is likely an issue with domain name and/or HTTPS conflicts. In your civicrm.settings.php file you define a CIVICRM_UF_BASEURL which may or may not include the www or https:// on the domain name. For example, it could be:


http://yourdomain.org/
https://yourdomain.org/
http://www.yourdomain.org/
https://www.yourdomain.org/


So my recommendation is to make sure that you forward the other 3 versions to the version you would like to use throughout the site. Then you may want to double check that the URLs set up in the CiviCRM Resource URLs page match (Administer > System Settings > Resource URLs).