This is not a normal behavior of RadMenu but the screenshot provided is not enough for a decision what is going on in your case. This kind of issues depend heavily on the layout of the page and the CSS in effect, that's why I asked for a live URL. If you can't provide a live URL then we will need a sample project, a Save As Web Page Complete from Firefox or at least some markup from that page in order to be able to help you. Thank you in advance for your trouble. I can convert this forum post to a ticket if you need the privacy.
the Telerik team