Dropdownlist getting full postback inside update panel

Question

Hey guys please tell me why i am getting full postback inside updatepanel my code given below

<asp:UpdatePanel ID="upanel" UpdateMode="Conditional" runat="server">

<ContentTemplate>

<h2>Other Information</h2>

<ul>

<li>

<asp:DropDownList ID="catdrop" ClientIDMode="Predictable" AutoPostBack="true" runat="server" OnSelectedIndexChanged="catdrop_SelectedIndexChanged" CssClass="form-control"></asp:DropDownList></li>

<li>

<asp:DropDownList ID="subdrop" runat="server" CssClass="form-control"></asp:DropDownList></li>

<li>

<input type="text" id="estd" placeholder="Year of Establishment(1994)" class="form-control" /></li>

<li>

<select id="emp" class="form-control">

<option selected="selected">Select no of employee</option>

<option>Less than 10</option>

<option>10-100</option>

<option>100-500</option>

<option>500-1000</option>

<option>1000-2000</option>

<option>2000-5000</option>

<option>5000-10000</option>

</select></li>

<li>

<input type="text" id="turn" placeholder="Annual Turnover" class="form-control" /></li>

</ul>

</ContentTemplate>

<Triggers>

<asp:AsyncPostBackTrigger ControlID="catdrop" EventName="SelectedIndexChanged" />

</Triggers>

</asp:UpdatePanel>