I think the extra year really mattered, and this is just me spitballing but I think once he started getting calls from a bunch of different teams his mind probably started to shift a bit. At one point he may have been set on coming back to Vancouver, but things change.
Friedman was saying that Toronto was also really in on Zadorov but that they weren't willing to give him the extra year that Boston did either. So for what it's worth, a team that arguably needed defense more than we did looked elsewhere as well. Which is saying something, because Toronto was trying to get both Tanev and Zadorov from Calgary last autumn, they just weren't willing to pay for retainment on both of them.
Zadorov spent quite a bit of time in Calgary, and a decent amount of time in Vancouver, he may have simply have been tired of the relentless Canadian spotlight. The Bruins are a big draw in Boston but it's not the same as playing in a rabid Canadian market. He can play the heel in Boston and still disappear into Boston in a way he probably couldn't do in Vancouver.
Taxes factor in too, because 5M in Boston, Vancouver, and Toronto ain't the same. 5M in Boston gets Zadorov 2.8M in take home money, 5M in Vancouver gets him just over 2.5M in take home money, 5M in Toronto gets him 2.35M in take home money. Now, creative accounting can offset some of that, but I do think it matters to agents and players.