Last time, it was climate change. From Smithsonian Mag:
A new theory links their fate to a meat-heavy regimen
While humans have barrel-shaped chests and narrow pelvises, Neanderthals had bell-shaped torsos with wide pelvises. The prevailing explanation has been that Neanderthals, often living in colder and drier environments than their human contemporaries, needed more energy and therefore more oxygen, so their torsos swelled to hold a bigger respiratory system.
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Alas, though fat is easier to digest, it’s scarce in cold conditions, as prey animals themselves burn up their fat stores and grow lean. So Neanderthals must have eaten a great deal of protein, which is tough to metabolize and puts heavy demands on the liver and kidneys to remove toxic byproducts. In fact, we humans have a “protein ceiling” of between 35 and 50 percent of our diet; eating too much more can be dangerous. Ben-Dor thinks that Neanderthals’ bodies found a way to utilize more protein, developing enlarged livers and kidneys, and chests and pelvises that widened over the millennia to accommodate these beefed-up organs.
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But as these mega-beasts [mammoths] vanished, the burly Neanderthals likely struggled to chase down smaller, swifter prey. Meanwhile, humans, with our narrow pelvises and agile forms, scampered into the future.
More.
A different theory puts it down to the fact that Neanderthals chewed more. And another one has it that they did not eat enough rabbits. A paleo-psychoanalyst claims they had large eyes and might have been weird loners.
Someone should make a film and see how many of these theories can be fitted into a connected storyline.
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See also: Neanderthal Man: The long-lost relative turns up again, this time with documents