I have found a bug in Ext.ux.ToolbarReorderer in webkit-based browsers (or at least Chrome and Safari on Windows and Mac).
When you attempt to re-arrange re-orderable buttons it will consistently refuse to allow you to move the buttons depending on the current location (either left or right). This does NOT seem to happen in FF or IE for whatever reason... so it appears to be browser specific.
I am able to consistently reproduce this bug with your own simple example:
Just load the page in Chrome or Safari, drag "Rating" right and then, after you've re-arranged it, attempt to drag it back to where it was before. It will not allow you to move it in either direction after that point. The other button can be re-arranged and, if you do that, it will sometimes allow you to drag the non-responsive button again...
I am running into this with a feature I am trying to implement right now with several more items (and this occurs midway in the items list too!), but this bug is obviously in your supplied and advertised UX!