Try setting the "IsSticky='False'" property for your RadAjaxLoadingPanel. If you reference this documentation page
, you'll note that "...The IsSticky property of the RadAjaxLoadingPanel defines where the panel will appear. By default, when IsSticky is false, the RadAjaxLoadingPanel will appear over the updated control. If you set IsSticky to true, the panel will appear where you have set it in the WebForm."
Hope this helps!