Yellow birch and old stump

What has happened to this tree? How come it seems to start off like a three-legged stool hovering over a stump?

Actually, this is not an unusual growth pattern for yellow birch, (Betula alleghaniensis). The seedlings like to grow in damp fertile areas. Sometimes a seed lands on the top of a rotten trunk,and the seedling grows there, putting down roots along the side of the trunk into the soil. Eventually the stump rots, and the tree is left standing clear above the remains of the perished stump.

This example was beside a trail in Arrowhead Park, near Huntsville, Ontario but the same growth pattern can also be seen at Backus Woods in SW Ontario.