When travelling trough my world I noticed that sometimes there are trees that have this narrow part on them. This happens when a branch intersects with a trunk of another tree. In this way the trees will be joined and they will both topple over when chopped down with an axe.

Both examples shown are with BOP trees, I'm not sure if it has ever happened with just DT.

This is a BoP add-on bug. The pre-generated models were recorded with the incorrect generation radius.

I have encountered this problem a few times too (and forced it to happen, in creative, but that doesn't count). Most of the time it involves mods, be it a new biome, bop trees, or a dimension like the twilight forest, but I think I've seen it once or twice with vanilla dynamic trees in vanilla biomes, etc.