Hi,
Make sure you do not manipulate DropDownList on server side code (.aspx.cs) again. Though you didnt post the full code snippet which is very helpful to find the actual problem, I think the code should be like this.
<asp:DropDownList ID="ddlCampuses" runat="server" OnSelectedIndexChanged="ddlCampuses_SelectedIndexChanged" AutoPostBack="True" AppendDataBoundItems="True">
<asp:ListItem Value="-1" Selected="True">Select</asp:ListItem>
</asp:DropDownList>
<asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ErrorMessage="* required" ControlToValidate="ddlCampuses" InitialValue="-1"></asp:RequiredFieldValidator>
Hope this will help.