Laura Erickson has written an excellent article about Snowy Owl migration and irruptions. Any given year (or two) represents too small of a sample size to be the last word on what these owls are doing, and why. I think Laura's more nuanced analysis is worth taking the time to read. While there's quite a bit we do know about these astonishing visitors from the arctic, just when you think you know the bird they go and change it up on you.
Link: Snowy Owl Migration: More complicated than most people realize
Snowy Owl © 2015 Mike McDowell