As Bumble Bee said, you most probably have either a function hooked to OnClientClose that submits the page, or you are calling something like GetRadWindow().BrowserWindol.location.reload() from the content page.
Check these help articles for more information on the subject:
You say however, that you want to have the value in parent's page code behind. To do this, you will have to make a trip to the server. Since you don't want to reload the page, the easiest way to do this is to use Ajax - and if you are using RadControls, there is a RadAjaxManager to help you in this task :)