I don't know if you've noticed, but when it comes to things, they "go." They may go well or go badly, of course. And occasionally they go very badly indeed. In which case English has a pile of idioms ready to apply to the disaster, a great many of them, unsurprisingly, involving the verb "go."
At times, then, things go south. I'm not sure why that should be — that south should be the direction of disaster, I mean. As a Canadian, I'm pleased, I guess, that nothing going south can geographically land in my country, but it still leaves me a bit puzzled. Perhaps the expression is an elegant variation on going downhill, which is slightly more understandable as a disaster movement, perhaps. Although, the expression "It's all downhill from here" suggests that coasting to the bottom isn't always horrible, as any kid on a bike will know.
As part of the general downward movement of disaster, things sometimes go pear shaped. This is down because the bulge is at the lower end; if it weren't, you'd have things going "mushroom." But why "pear shaped" instead of "foot shaped" or "pyramid," for instance? I resist the thought that it's because of the migration downward of avoirdupois in some as a function of aging. No, I side with the OED who claim that it's an RAF expression (the earliest use is from the 1980s) for a cock-up. There's something here perhaps: I mean, you can see how down would be a bad direction much of the time if you're in an air force. And as for the "pear," some suggest that it means to describe a badly executed aerobatic loop, though any pilot who could limn one of those would be surely have to be more expert than the average, I'd have thought.
Then things go down the tubes for some. This is easier to see as a disaster plan: that way lie ever bigger pipes until before you know it you're being dealt with in some effluent treatment plant, than which few things could be worse—except perhaps finding yourself emitted out to sea, as you still might in Halifax or Victoria, I gather.
Lurking beneath all of these idiomatic expressions and metaphors of descent has to be, surely, the Big Downer, the Ultimate Disaster—that visit to the underworld. Because, let's face it, things may in fact go to hell. If it's only a little hell, or perhaps the outskirts of the inferno, we'll make light of it as we watch things go to hell in a handbasket. The alliteration helps here, speeding things along; as does the basket, if you think about it, because once in it, a thing can offer no more resistance than could your groceries. And it has a nice domestic ring about it, which could well be the sound you'd want to hear if you were tucked up next to the devil's food cake and on your way . . . below.