postback of dropdown causes disapear other required fied validator Error messages

Hi, I have a dropdownlist and a textbox, each of them has it's requiredfieldvalidator like this
<form id="form1" runat="server"> <asp:DropDownList ID="drp1" runat="server" AutoPostBack="True" onselectedindexchanged="drp1_SelectedIndexChanged" > <asp:ListItem Value="-1">--Please Choose--</asp:ListItem> <asp:ListItem Value="1">Sale</asp:ListItem> <asp:ListItem Value="2">PreSale</asp:ListItem> </asp:DropDownList> <asp:RequiredFieldValidator ID="rfv_drp" runat="server" ErrorMessage="RequiredFieldValidator" InitialValue="-1" ControlToValidate="drp1" ></asp:RequiredFieldValidator> <br /> <br /> <asp:Button ID="Button1" runat= ...