No idea, wild guesses incoming... Early humans (or proto humans) might not know that it was warmer in the south or colder in the north, across vast areas. Some

No idea, wild guesses incoming... Early humans (or proto humans) might not know that it was warmer in the south or colder in the north, across vast areas. Some days are colder than others, even if you stand still. Add to this the effect of changing seasons through a year, some years being colder than others anyway. You could migrate North slowly as Winter turned to Summer. Then camp for the winter and migrate back. And their migrations might have moved slightly further north year by year. They would have to track temperatures from winter to winter, without a calendar, thermometer, notebook, pen, map, compass and perhaps even an enquiring mind. I don't think that humans decided to move to a colder place or even realised that their movement was taking them somewhere cooler. Ultimately, they would have followed their food wherever it went. You might be better off asking why a caribou herd migrated further North. Who knows? Better grass? Less competition? Fewer predators? Lower sea level opening a path to new grassland?