It has something to do with the Z-Index of the dynamic menu.
It's a known problem, the fix is so.
Create a style similar to this:
.linkedMenu
{
z-index: 100;
}
and add to the menu control (
<asp:menu xmlns:asp="#unknown"></asp:menu>
)
the following line
<DynamicMenuStyle BorderStyle="Ridge" BorderWidth="3px" CssClass="linkedMenu" />