agreed...some sort of physical damage years ago. Lightning is a possibility. Maybe another tree fell against it?
As others have said, bacterial wetwood. Not going to hurt the health of the tree.
If I had to guess, there is minor decay associated with that. tap around with a mallet to get a sense for how extensive that is. If the tree falls, will it hit anything? If it will, probably worth having somebody versed in risk assessment evaluate the tree. Not the biggest advertiser for "we remove trees cheap" - I already know what their answer will be. You'd be looking for a paid consultation/tree risk assessment.