As Bonsai enthusiasts will tell you, a tree will continue to grow in whatever direction it can when something has caused it to be unable to go up. They only go straight up because that's the shortest route to the sun. It could be that something struck the tree and caused the trunk to bend out that way and so it simply grew thicker branches straight up along the now bent trunk. You could even do it on purpose to fuck with people if you wanted to.

It's thought that The Bent Forest was done on purpose by people to create timber for boats, rather than having to cut down straight trees and bend them.

Nice!