I am personally leaning (pun intended) on Phototropism as the explanation.
It is very interesting to read about it any why it's not happening with all of the plants in the lower or upper hemisphere more often.
I will investigate it further, if I don't forget. My country also has this long weekend so I should have time :D
I'm also in favor of phototropism. It makes sense that they'd want to maximize the available sunlight.
Even when there is nothing blocking the sunlight? As if the trees follow the sun's route in the skies?
I think magnetic forces may have to do something. I guess we'll have to wait some more years...
If you find anything new, let us know ;)