In your code, you are checking if the page is not a postback. If its not a postback, it will execute the code in the if statement.
Therefore, if you're calling the above method after the selected index change, make sure to remove the if condition. See
http://msdn.microsoft.com/en-us/library/system.web.ui.page.ispostback.aspx
After the binding, I would suggest to set the selected index to -1 in order not to have any item selected.