It is during a time of chaos that wars take place. More likely starving Sapiens were once again goaded by starvation into launching an attack on the highlands where Neanderthals kept their animals.
Agriculture would be gone during this period but herders, as always, would continue to persist. You can shepherd your flock beneath the roof of a cave in almost any conditions. Neanderthals had a far stabler long term plan for survival than Sapiens ever had and as always was probably stocked with provisions years in advance. He would have presented an irresistible target during a major famine to the Sapiens horde.
Abel was a herder. Cain was an agriculturalist. Cain killed Abel because his labors were rejected (found insufficient) by God.