The post is a travelling position, which means Hawking can stay on as an emeritus professor at Cambridge while making regular visits to Canada.
A friend says it's because the Perimeter Institute's new director, Neil Turok, knows Hawking reasonably well. Sounds right.
People here could provide Hawking with a short list of months in which he might be best to avoid the PI. (About six of them actually.)
Space exploration: Stephen Hawking urges, Boldly go ... but why, exactly?
Stephen Hawking, miffed over science funding cuts, to move to Ontario, Canada?
Who reads popular books on cosmology? Well, almost everyone who actually reads, it seems
Study: Sun not special, therefore alien life should be common?