Nancy Reading wrote: I suspect it is just the loss of leading branches, possibly from disease or browsing animals.
I would suggest several of the ones with sharp corners in Logan's pictures would have been deer browse in my ecosystem.
The more gentler curved ones might well have been changes in gaps in the canopy allowing light to come from different spots over the years.
There are absolutely places where humans intentionally impacted the shapes of trees when they were young, particularly for the boat/ship building industry. Why have to cut wood to the shape you want when Mother Nature will do it for you with just a little "guidance"!