I've tried to expand rich:tree nodes in Richfaces Showcase. My deployment on Openshift is available here and the WAR files are available here in JBoss repository. It is working for me. Could you create a new issue on https://issues.jboss.org and provide more information to reproduce the bad behaviour?
I found out what was wrong.
I used <center></center> somewhere else in my .xhtml file.
After I removed the center tags, the tree works normally.
Not sure why though.