As mentioned by schroet, this is caused by modal window, see ZK Window Javadoc
As a workaround, you can modify the doPickDocumentRSN method in PopUpViewModel.java as below:
public void doPickDocumentRSN(@ContextParam(ContextType.VIEW) Component comp) {
Map maps = new HashMap();
maps.put("PARENT_VIEW", comp);
maps.put("peopleTabArg", new TabArguement(TabArguement.TabMode.PICK));
UserInterfaceViewModel.addAnyTab(maps, "Tab2", "tab2.zul");
comp.setVisible(false);
}