Modal Popup Extender Displaying unexpectedly

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C#

ASP.NET

I have a modal popup 'wucLoanRepayment.ascx' on a parent form named DeliveryOrder.aspx.

on the modal there is a button btnSave for saving operations on the modal. there is another button btnDone which is used while Modal needed to be close (I have not used the CancelModalID property to close modal) and some data returns to parent form. everything is working fine the btnSave, btnDone and returning data to form.

The problem occurs when I have closed the modal with btnDone after save operation in the modal, if there is any kind of postback event occurs in the DeliveryOrder.aspx the modal popup appears though I dont want it to be appeared in those post backs. the modal displays for the first time of postback for the next postbacks it does not appears.

Why the modal is appearing like this?

N.B. I have used ASP.NET AJAX in every operations.