How do you fuck something like that up?
All too easily it seems; my first MacBook Pro power lead caught fire a few years ago as well. This was the low-voltage (hence high current) end, though: in their quest to make everything thin and light, the cable was thin and flimsy, so one of the braided conductors frayed after a while. More current going down a thinner wire meant more heat - which softened the remaining copper and made the problem worse, until arcing started and I got a micro-firework display on my desk. (One of is successors managed to melt the plastic in the plug, that didn't make me happy either!)
On the mains end, even a hefty (for laptops) 300-odd watt PSU is only 3A from a US outlet, half that on the higher voltages elsewhere - usually easy enough to deal with, but one sloppy connection and you can get a tiny point getting very hot indeed. It's worse on the low voltage end: a single cable possibly carrying 20 or more amps, while getting rolled up, folded and stood on in transit, designed to be very light weight - yet also done on a budget. As soon as you start trying to shave weight and cost, I suspect it's all too easy for a wire to be just slightly too thin for the current, or a connection to be a little bit too weak for long term mobile use.
If you were building a high school or college electronics project and said you planned to run laptop currents and voltages through such thin wires and tiny connectors, you'd probably be told off or marked down - but commercially, thin, light and cheap trump safety margins and robustness.