Dropdownlist keeps on refreshing

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ASP.NET

Hi

I have a dropdownlist, every time I select an option it refreshes to the first selected value. What might be the problem?

protected void Page_Load(object sender, EventArgs e)
        {
            if (!Page.IsPostBack)
            {
                populateDDlEmployeeIDOnpageLoad();
                loadgrdShowSelectedappointmentOnPageLoad();

            }

        }


        #region Load ddlEmployeeID On page Load
        public void populateDDlEmployeeIDOnpageLoad()
        {
            ArrayList staffList = new ArrayList();
            systemBusinessLayer = new BusinessLayer();
            
            staffList = systemBusinessLayer.loadEmployeesWithAppointments(date);

            ddlEmployeeID.DataSource = staffList;
            ddlEmployeeID.DataTextField = "EmployeeName1";
            ddlEmployeeID.DataValueField = "EmpRecNumber1";
            ddlEmployeeID.DataBind();
            ddlEmployeeID.Items.Insert(0, ".:Select Employee:.");



        }
        #endregion


        #region A method to Load grdShowSelectedappointment On Page Load with all appoinments
        protected void loadgrdShowSelectedappointmentOnPageLoad()
        {
            string date = DateTime.Now.ToShortDateString();
            DataTable dt = new DataTable();
            systemBusinessLayer = new BusinessLayer();

            //  dt = systemBusinessLayer.grdLoadSpecificEmployeeOnPageLoad(date);
            dt = systemBusinessLayer.GetAllAppointments(date);

            grdShowSelectedappointment.DataSource = dt;
            grdShowSelectedappointment.DataBind();

        }
       
        #endregion

public void getEmployeeID()
       {
           dTable = new DataTable();
           systemBusinessLayer = new BusinessLayer();
           dTable = systemBusinessLayer.GetAllEmployeesInfo();
           foreach (DataRow dRow in dTable.Rows)//Go through each row inside datatable
           {
               string fullname = dRow["Fullname"].ToString();
               if (ddlEmployeeID.SelectedItem.Text == fullname)
               {
                   Session["EmpRecNumber"] = dRow["EmpRecNumber"].ToString();
               }
           }
       }
       protected void ddlEmployeeID_SelectedIndexChanged(object sender, EventArgs e)
       {
           getEmployeeID();
           if (DatePicker.SelectedDate.ToShortDateString() == "1/1/0001")//if date is not selected
           {
               if (!Page.IsPostBack)
               {
                   MessageBox.Show(Session["EmpRecNumber"].ToString());
               }
           }
           else
           {
               MessageBox.Show("Invalid selection");
           }
       }