More than likely old age caused by a failing root system. Grinding stumps below ground especially if you can get at least a foot below grade tells the story in detail of a trees demise if you know how to read wood. Seen many a tree with no apparent above ground symtoms for having dead limbs and general overall poor health till they are cut down and you grind the stump. The answer is then obvious.
Trees don't last forever. No sense in getting sentimental about just bite the bullet and get the dirty deed over and done with and make some good firewood or lumber out of them.
The plain Jane common ash borer can also take an ash tree down over time.
We had an over mature Red Oak just fall down into the field last fall. Like you AL, I would have rather had the tree stand but the root system coudn't keep the tree standing anymore and just fell. Fell in a nice spot too. Nice open field, crops were out, no brush or undersory small trees to contend with. Easy pickin's. Nice winter project and some good firewood for us. Darn farmer who rents from us just started helping himself to it. I was pissed. Took all the nice stuff and left the junk without asking thinking we didn't have a use for it. DRR. I made him clean up the mess as I told I would charge him for the wood if he didn't as their was no way I was gonna clean up after them. No sir, no way.
I know it was just a tree, but damn it, it seems like any tree on the ground especially a hardwood like that is fair game to anyone with a chainsaw. I thought this one was safe because it was on our property and felt there was no rush as the crops were out and no one knew it was there. Wrong.
Shoot, even some of our not so nice neighbors decided to help themselves last week to our split wood that we sell for campfire wood to have a little backyard party. Didn't even ask just took it after dark. Would have given them the wood if they had asked but instead just stole it thinking we have so much we wouldn't miss it or that we would mind. I was again pissed off when I saw that. Not the first time something has just disappeared either.