By design, RadSplitter preserve the state of the expanded/docked sliding panes over postback. If you need to avoid this behaviour you need to manually reset the ExpandedPaneId property of the sliding zone, e.g.:
sender, EventArgs e)
I hope this helps.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed