There's something in Canada called an RCA (retirement compensation agreement) which allows players to defer their tax obligations on up to 50% of what they are earning in a given year. Half goes into a refundable tax account and the other half goes into a self-directed account. The outcome of this is that they pay 50% of their tax obligation, per year, on half of their earnings - while the other 50% is held and invested in that self-directed account.
The reason I bring this up is the major caveat with the RCA: upon redemption, they need to break all their ties with Canada for a minimum of 18-24 months. They then pay a flat 25% tax on those self-directed, deferred-tax earnings (which have been growing, tax free, for x amount of years), and after that term are able to repatriate/repurchase their real estate/small businesses, etc. I know for a fact that Iginla still "owns" two lakefront properties nearby me in the Okanagan, but the title is held in his parent's names. I see him at my course quite a bit in the summer.
Anyway, the baseline assumptions in an RCA point right to why he'd settle in Boston.
Alan Walsh went into detail on this on Chiclets a while back:
Savvy tax planning man - big boon for the wealthy. The napkin math on this is outrageous and I can run it in a bit, but it's a truly enormous cost savings over the course of his career - many sheets worth.