I am missing an updatepanel. Put the textbox inside an updatepanel like this, with the timer as trigger and try again.
<asp:Timer ID="timer1" Interval="8000" runat="server">
</asp:Timer>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:TextBox id="AllChat" runat="server"
type="text" class="style15" TextMode="MultiLine" Height="172px"
Width="316px" ReadOnly="True" CssClass="style2"></asp:TextBox>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="timer1" EventName="Tick" />
</Triggers>
</asp:UpdatePanel>