I sliced a bit off the side to show what it looks like under the bark. Surprisingly the bark actually isn’t any thicker at the thick base than it is on the narrower part.
And now for the answer!
This is an ungrafted feral tree. Prunus avium is actually somewhat “invasive” in my area and in the spring when they are blooming you will see them all over in the woods. Some of them tend to grow as solitary trees and some seem to make clonal colonies. This particular specimen is part of what appears to be a clonal colony with every trunk producing small, red blushed-yellow cherries (most other trees in the area ripen red). I don’t typically see this thickened base when I come across them, but for this specific colony the thickened base seems to be characteristic. Because there is no sign of this being caused by any disease or fluctuation of soil level (which I can vouch for given that these are at my childhood home), I have to assume this trait must be from a genetic predisposition.