There are other factors, but not wanting to get up is a significant one.
We put down my wife’s childhood horse this May. She was 26 years old, had been unrideable for 8 years, had trouble getting up (often needed assistance) for 4 years. She was a tall saddlebred. Her conformation was very ‘leggy’, which was part of the problem. She was way down in the pasterns. We had to feed her a lot of sweets & alfalfa to maintain weight. A couple times in the past she had lain down for extended times and suffered pulmonary edema and would ooze a gallon of mucus from her nose after we pulled her up.
The day we ‘decided’ started with me having to pull her to her feet, and moments later she went back down. She didn’t really try to stand. So I let the herd into the fresh grassy pasture, and the old mare didn’t get up. I fetched the wife and we both pulled the old mare up. Then the mare walked into the center of the pasture, and plopped down again. This time we couldn’t raise her. The pasture is a major treat, and she wanted none of it. I brought her some sweet feed and she ate laying down. She wasn’t off her feed. She wasn’t in colic. She was saying she was done.
We called the vet, he pumped in a couple 60cc syringes of the nigh-nigh-juice and she passed in the middle of a grassy pasture on a May morning. It was fortunate because the disposal guy had plenty of room to remove her, unlike many who die in the back of a stall inside a barn. The poor mare also didn’t have to endure another miserable Phoenix summer. I hate putting my horses through the Phoenix summers.
BTW, the horse carcass disposal guy charges a minimum of $450/horse, and gets 10-12 horses spanning a 15 hour work day. His overhead is a 20 year old Super duty diesel and a heavy duty trailer with a hydraulic lift. It’s dirty work, but he’s making $$$.