As the tree becomes very mature the need for this volume of rigidity diminishes and heartwood decay begins to takes place. This is a natural process that all old trees go through.
The decay fungi breaks down the structure of the wood, which then enables the tree to reabsorb the nutrients locked away in the heartwood for centuries.
This is done by producing internal aerial roots which grow inside the decaying wood. These can become visible if the decay process extends all the way to the outside in fragmented sections of the trunk.
The decaying wood acts like a slow release fertiliser for the tree.