The great majority of animals on the plains migrated to the South starting in the fall of the year when herds sensed instinctively that it was time.  The snows in the Northern plains could get brutal with high winds and deep snow.  Not to say there was not snow in the Southern plains as well but everything was milder and conditions presented the best odds at survival.  Buffalo never played the odds of course but just did what their instincts demanded.   It was natural.  It was a cycle that went on for thousands and thousands of years, uninterrupted by human machinations and the unfailing lurk and creep of civilization in the form that is common today.   Everything has it's time and time stops for nothing.