Expand Root MenuItem on Click Only
You can use the ClickToOpen property to specify that menu items do not expand when the mouse hovers over them until the user clicks the menu with the mouse. When ClickToOpen is True, the menu does not expand until the user clicks on a root item, at which point the root item expands. Once clicked, the menu expands and collapses as normal, until the user clicks again (either on a menu item or outside the menu).
This example will show how to prevent expanding of the other root items on hover.
<telerik:RadMenu RenderMode="Lightweight" ID="RadMenu1" runat="server" ClickToOpen="True" OnClientMouseOver="OnClientMouseOverHandler">
function OnClientMouseOverHandler(sender, eventArgs) {
if (eventArgs.get_item().get_parent() == sender) {
sender.set_clicked(false);
}
}