C# - Timer Results In Updatepanel Gives Full Postback

I have a quite strange situation where I have the following code:

<asp:Timer ID="GameClock" runat="server" Interval="5000" Enabled="true"
ontick="GameClock_Tick">
</asp:Timer>
<asp:UpdatePanel ID="ItemsUpdatePanel" runat="server" UpdateMode="Conditional" ChildrenAsTriggers="false">
<Triggers>

NOTE THE TRIGGER IS COMMENTED OUT

<%--<asp:AsyncPostBackTrigger ControlID="GameClock" EventName="Tick" />--%>
</Triggers>
<ContentTemplate>
<asp:ListView ID="PlayerItems" runat="server" GroupItemCount="7"
onitemdatabound="PlayerItems_ItemDataBound">
<LayoutTemplate>
<table border="1" cellpadding="2" cellspacing="0" runat="server" id="tblProducts">
<tr runat="server" id="groupPlaceholder">
</tr>
</table>
</LayoutTemplate>
<ItemTemplate>
<td id="Td1" runat="server" style="vertical-align:top; text-align:left; height:100%;">
<div>
<div id="category" runat="server">
<asp:Panel ID="ItemPanel" runat="server">
</asp:Panel>
</div>
</div>
</td>
</ItemTemplate>
<GroupTemplate>
<tr runat="server" id="productRow">
<td runat="server" id="itemPlaceholder"></td>
</tr>
</GroupTemplate>
</asp:ListView>
</ContentTemplate>
</asp:UpdatePanel>

This gives a quite strange result: Every 5s my whole page gives a full postback. When I comment in (activate) the asyncpostbacktrigger, the updatepanel does not give a full postback. In the PlayerItems_ItemDataBound I have the following code (which, I do think, do not matter):

protected void PlayerItems_ItemDataBound(object sender, ListViewItemEventArgs e)
{
if (e.Item.ItemType == ListViewItemType.DataItem)
{
ListViewDataItem dataItem = e.Item as ListViewDataItem;
if (dataItem != null)
{
BaseItem myItem = dataItem.DataItem as BaseItem;
Panel itemPanel = new Panel();
Literal firstLiteral = new Literal();
firstLiteral.Text += "<div id='smoothmenu1' class='ddsmoothmenu'>";
firstLiteral.Text += "<ul>";
firstLiteral.Text += "<li><img src='Images/Game/Items/" + myItem.ItemImageUrl + "' />";
firstLiteral.Text += "<ul>";
// Add all events bound to item into contextmenu
itemPanel.Controls.Add(firstLiteral);
foreach (Delegate del in myItem.Actions.Items)
{
Literal firstItLit = new Literal();
firstItLit.Text += "<li>";
itemPanel.Controls.Add(firstItLit);
MethodInfo methodInfo = del.Method;
string commandName = myItem.ItemId + "|" + methodInfo.Name;
LinkButton btn = new LinkButton();
btn.Text = methodInfo.Name;
btn.Click += new EventHandler(btn_Click);
btn.CommandName = commandName;
itemPanel.Controls.Add(btn);
Literal secondItLit = new Literal();
secondItLit.Text += "</li>";
itemPanel.Controls.Add(secondItLit);
}
Literal btnLiteral = new Literal();
btnLiteral.Text += "</ul>";
btnLiteral.Text += "</li>";
btnLiteral.Text += "</ul>";
btnLiteral.Text += "</div>";
itemPanel.Controls.Add(btnLiteral);
Panel panel = (Panel)(e.Item.FindControl("ItemPanel"));
panel.Controls.Add(itemPanel);
}
}
}

When I create a NEW updatepanel, ItemsUpdatePanel1, it does not fire a full postback without the timer. I can even start copying items from ItemsUpdatePanel to ItemsUpdatePanel1, and suddenly the full postbacks happen. I tried 2 seperate times, and they started happening at different times.