The problem is due to the IE native focusing behavior of editable DIV elements. 

You can workaround this problem by overriding the setFocus method and implement an if statement that does not focus the content if the  AutoResizeHeight is enabled:

<telerik:RadEditor ID="RadEditor1" runat="server" AutoResizeHeight="True" ContentAreaMode="Div">
</telerik:RadEditor>

<script type="text/javascript">
	Telerik.Web.UI.RadEditor.prototype.setFocus = function () {
		try {
			if ($telerik.isIE && this.getSelection().isControl()) return;

			var area = this.get_mode() != $T.EditModes.Html ? this.get_contentWindow() : this._getTextArea();
			if (this.get_contentAreaMode() == $T.EditorContentAreaMode.Div) area = this.get_contentArea();
			if (area && area.focus && !this.get_autoResizeHeight()) area.focus();
		}
		catch (e) { }
	};

</script>