ListSearchExtender caused 2 postbacks instead of one

When I change selected item in DropDownList, then begins asynchronous postback and some UpdatePanels are reloading. I put one break point in the Page_load event. This break point activated once for each time when I changed selection in the DropDownList. So everything was ok.

 <asp:DropDownList ID="ddl_otdel" runat="server" AutoPostBack="True" DataTextField="otdel_name" DataValueField="id" OnDataBound="ddl_otdel_DataBound" OnSelectedIndexChanged="ddl_otdel_SelectedIndexChanged" />  <%-- <ajax:ListSearchExtender ID="listSearch_ddl_otdel" runat="server" TargetControlID="ddl_otdel" PromptCssClass="ListSearchExtenderPrompt" PromptText="Введите начало строки..." > </ajax:ListSearchExtender> --%>

After that I decided to use ListSearchExtender with my DropDownList. Since that moment my break point (in Page_load) began to activate twice for each time when I changed selection in the DropDownList. When I commented ListSearchExtender then everything became ok. Why there are 2 asynchronous postback instead of one?

May be my question isn't correct. Maybe I don't understand many things. But there is a problem that in some conditions ObjectDataSource executes SelectMethod when it's not necessary and page is going through server-cycle twice during one postback.