I had an older version at home. And the shifting happened there as well.
It also happened on the tree trunk itself.
I'm baffled by why this happens, as I've not had this problem before. I wonder if it could be related to geometry with hidden and/or soften lines? (Wildly guessing here. Only semi-relation I could think of.)
Yea, the tree leafs are thrown on very oddly. But I expect no-one would make a close up of such a low poly tree. I've used it though to test out the V-Ray render plugin, so I've added some more details to it. extruded the single line branches etc.
At least now I know that it wasn't just me doing something weird in SU.
I might try some more scenarios and see if I can track down the triggering factor for this behaviour and submit a bug report.
Thanks for your help.