Rot can attack and remove the heartwood. This usually leaves a burnt look on a hollow inside. The center of a tree trunk is not the living part. The way the bark came off around the base, looks like the outside wasn’t doing too well either. The rot probably took out too much of the center structure for the tree to support itself. I think the outer living greenwood has a harder time pulling up water and nutrients from the roots when the interior of the tree is exposed to open air as well. With the tree so close to pavement, it likely struggled to get enough moisture and maintain a healthy root system too.