Two become one

This is two trees - I wonder if it's coincidence that sometimes trees kind of merge into a single natural shape or if there's a science?  

I guess if they run out of room on one side, then this would be the result, but you'd expect a bit more randomness.  One to grow taller perhaps.

Or maybe there are so many trees with so many permutations, that the laws of chaos mean that you're bound to get some that look like this.  

I remembered some others so went hunting through my dreadfully oversaturated old blips to find some examples;

Here

Here