Extreme obesity

I think in most cases, it is gradual. They start out with little aches and pains, then it gets worse. As time goes on, they can walk less, and prefer not to walk.

From a few cases I've heard of, I suspect that most often immobility happens after an accident of some sort. As mentioned, you can build the strength to carry a lot of weight, although it will get more and more tiring and painful. But one fall where you twist and ankle and mess up your knee, and you could have a hard time recovering (really can't put a massive weight on the injury, so can't keep up that strength, and if you were marginal in the first place you might fall below any sort of functional mobility)