I think the fat part that fans out near the root is actually under a bit of tension with the sides pulling outward from the middle. Do a longitudinal slice (i.e. in the direction of the fibers) down the middle and you’ll see the stalk split apart to the left and right of the cut.
When you cut crosswise from the convex part there’s less material holding the middle together so it wants to split. Cut from the back/concave side and you’re first severing the parts that are exerting the outward force, so no splitting.
I just made all that up but it seems to align with my observations.
Ha! Science and vindication!