@meoralis It started with a wound deep into the bark (possibly a cut heart). The tree then tries to protect themselves, what happens is called 'compartmentalization', a kind of self-sealing. Callus grows around the wound (bulbous parts).
Later, decay fungi will conquer these parts (probably already in this tree), and it takes some more years until the tree dies. Here it's explained with photos (pdf):
https://utia.tennessee.edu/publications/wp-content/uploads/sites/269/2023/10/SP683.pdf