This is the general problem face by developer even I also suffered from this problem. In my scenario, I was using RadioButtonList inside ModalPopupExtender. what i was facing when i changed the selection of RadioButton option, it was closing the Ajax ModalPopupExtender.
So what i did to prevent closing of ModalPopupExtender.
I put the modalpopupextender inside the UpdatePanel and in RadioButtonList1_SelectedIndexChanged event I called the ModalPopupExtender show method. like that
protected void RadioButtonListShippingMode_SelectedIndexChanged(object sender, EventArgs e)
{
ModalPopupExtender1.Show();
}
It really solved my problem.