I'm gonna guess Axford wanted/asked for it. With Hand and Soria being acquired, Pearson being converted to a reliever and one of Matz/Stripling probably also going to the pen, they probably didn't have space for Axford on the roster anymore. Having built himself back up as a professional pitcher, he probably didn't want to just call it quits because the Jays didn't have roster space. This way, they move him to the team he's most familiar with, at a time where they're bonafide contenders.