CKEditor delete session in IE

I found out that ckeditor doesn't delete the session when I click on image icon in toolbar and image popup window is shown.

But I found out that when I click on image icon in the toolbar ckeditor start reading my index.php (without query_string) file in the background and there my program delete session because the query_string iz not set. This happens only in IE.

It is very strange why ckeditor run index.php, doesn't it?