According to the WAI-ARIA specification, the  Shift+Tab in Rich Text Box should focus the toolbar if the cursor is in the content area.As a side effect the Tab functionality get executed instead of Shift+Tab.

The behavior is correct in FF and IE, but not in Chrome

Possible workaround is overriding the InsertShiftTab command with this function:

<telerik:RadEditor runat="server" ID="RadEditor1">
</telerik:RadEditor>

<script type="text/javascript">
	Telerik.Web.UI.Editor.CommandList.InsertShiftTab = function (commandName, editor, oTool) {
		var oParent = editor.getSelectedElement();
		if (typeof (oParent.tagName) == "undefined")
			oParent = oParent.parentNode;

		var tdNode = Telerik.Web.UI.Editor.Utils.getElementParentByTag(oParent, "TD");
		if (tdNode) {
			Telerik.Web.UI.Editor.Utils.MoveToPreviousCell(tdNode, editor);
		}
		else {
			setTimeout(function () {
				var toolsArray = editor.get_toolAdapter().get_tools();
				var lastTool = toolsArray[toolsArray.length - 1].get_element();

				if (document.activeElement !== lastTool) {
					lastTool.focus();
				}
			}, 0);
		}
		return false;
	};
</script>