There's a lot of things that can spontaneously kill anyone. Random blood clot in the wrong place, for example. And old age predisposes you to having those kinds of events and also not recovering from those kinds of events.

E.g., say an otherwise healthy person gets an active heart attack. Some people, for unknown reasons, their heart will try to power through the problem and keep pumping. It hurts and is not good obviously, but it could buy them time to get treatment. But strangely, some people's hearts will encounter this problem and just stop functioning. We don't really know why every time. It's partly genetic luck though.

Otherwise, death is usually from a chronic condition of some kind, not entirely unexpected. It's a process. You don't just die all at once. Your body exhausts itself slowly building from the primary chronic problem, like a bad liver. And it's in your sleep because eventually, your body doesn't have the resources to actually keep your brain fully conscious.

For example, my grandma was around for two weeks after we all agreed to stop the main healthcare and go for hospice instead. She was awake and "fine" in the beginning. no real bad pain or anything. You wouldn't know just from looking at her that she was dying. we could chat. But she no longer felt hunger and her body didn't really produce waste normally. as the time went on, she spent more time asleep. And eventually she didn't wake up much at all.