The population pressures on this bird that are causing these changed behaviors aren't affecting all birds so it can't just a simple case of temperature shifts affecting tolerances. Also measured temperatures in the northern snowy owl habitats have been increasing, so it's not getting colder.
One hypothesis is that the warmer weather in northern Canada is creating larger populations of lemmings, which allow larger snowy owl broods, creating an overall larger population. When it's time to migrate, the larger populations are forced to disperse further south by territorial pressures by the larger population.
This article was discussing the Cornell Labs biologist disputing the idea of food abundance increasing population, and suggesting that the warmer Canadian temperatures that have been measured in the last few years are disrupting northern habitat forcing the birds south.
Either of make more sense and also have the advantage of fitting some measured data, though it's still something of a mystery.