This problem might occur if by some case the RadEditor's CSS files are not loaded correctly. Could you please register the external RadEditor skins as it is shown in this KB article: Registering an external skin of RadEditor
You can also take advantage of the recently introduced CDN
support which will allow you to load the controls skins from the Telerik CDN. Telerik CDN is hosted on the Amazon CloudFront service. You can find more information about our CDN support here
By loading the external or CDN skins you will ensure that the editor will load its skin.
In addition, if this problem occurs only under Internet Explorer the most possible reason is that there are more than 31 CSS files registered on the page (including the ones registered by any controls). More detailed information and possible solution on this issue is available in the following blog post:
Internet Explorer CSS limits
the Telerik team