The RadDock control cannot be expanded when it is set in lightweight render mode and its property Collapsed is set to true.

You can avoid this issue by setting the dock in collapsed state via its client-side API:

<script type="text/javascript">
    function OnClientInitialize(sender, args) {
        sender.set_collapsed(true);
    }
</script>
<telerik:RadDock runat="server" ID="RadDock1" OnClientInitialize="OnClientInitialize">
</telerik:RadDock>