The native ASP.NET controls are capable to preserve their values when using back button of the browser. In the RadEditor control this appears to be broken since 2014 Q1. 

This issue is encountered due to bug fixing related to a memory leak in IE8. You can resolve this by using the old functionality for content preserving and verify if everything works as expected under IE8.

The following markup example shows how to roll back to the old behavior:

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

<script>
    Telerik.Web.UI.RadEditor.prototype.set_contentHiddenTextareaValue = function (htmlValue) {
        var utils = Telerik.Web.UI.Editor.Utils;
        $telerik.$(this._contentHiddenTextarea).val(utils.encodePostbackContent(htmlValue));
    };
</script>