The DropDownTree pop-up is not positioned properly when the <form> is wrapped inside a relatively positioned element with fixed width.
The issue can be reproduced with a similar configuration:
<div style="width: 700px; position: relative; left:300px">
<form id="form1" runat="server">
<telerik:RadScriptManager runat="server"></telerik:RadScriptManager>
<div class="position">
<telerik:RadDropDownTree runat="server" ID="ddt1" CssClass="dropDownTree"></telerik:RadDropDownTree>
</div>
</form>
</div>