Stag, bone, plain wood and ivory are all fairly delicate materials that, unless worked correctly often crack. That cracking may not show up for years, but in your case I would bet that the pin was hammered(peened) to hold the handle down. What this causes is pressure working against the handle material which eventually, especially if the material shrinks a little with age, caused the splitting. Pins used in those materials should be glued in place, never hammered. The one thing you can try is buy a bottle of super glue at your local store and place a few drops on a piece of waxed paper. For some reason SG stays fluid for a long time that way. Get a fine pin and gently work some of the glue into the crack if you can. Let it dry, and sand gently with a bit of high grit (around 400) sand paper. If the crack is too fine to take any sort of glue, I'm afraid you'll just have to live with it, and realize the cause of it. Maybe someone else has a better idea.
