DropDownList SelectedIndexChanged event not fired when programatically setting SelectedValue

Question

Hi,
In asp.net web application we load the dropdown list boxes in the page load.  We than set the SelectedValue of each dropdown list box to an object from session.  We have event handlers for many of the SelectedIndexChanged events of the drop down lists boxes, but these do not get executed when we set the selected value programmatically. But it get raised when a user manually changes the drop down list selected index. why it is happening so?
Any idea? please help!!!!