My understanding is that they are being driven south to find food in sufficient number that the nanulak (polar/grizzly hybrids) population has become more noticeable.
Bud Grant: 'As one man can love another, I love you Sid Hartman' http://www.startribune.com/bud-grant-as-one-man-can-love-another-i-love-you-sid-hartman/391069961/