When the panel first shows up (horizontally), it doesn't set the _NET_XFCE_TRAY_MANAGER_ORIENTATION property. Only when changing the orientation of the panel, the property will be set. Also the system tray should support the _NET_SYSTEM_TRAY_ORIENTATION property, as specified in the system tray spec 0.3.
Ouch, _NET_XFCE_TRAY_MANAGER_ORIENTATION was a typo, should've been _NET_SYSTEM_TRAY_ORIENTATION, good catch! Initial orietation should also be fixed in revision 29684.