I stumbled upon a birch growing in sandy soil in a coniferous forest in central Russia. It looks like over time the tree trunk got bent towards the trail and one of the branches became the new trunk as it now grows straight up, whereas the old trunk is pointing sideways.1
The tree isn't dead, I visited that place during summer time and it was covered with green foliage.
I'm wondering what's the name of such phenomena, how common it is and what usually causes the tree "to change it's mind"?
1. I apologize in advance if the terminology I'm using is inaccurate, so feel free to edit my question.