it's more of an occam's razor thing imo.
we are already aware of a guy who was arrested for having terabytes of classified info, who worked for TAO. i'm not sure how it would have gotten from martin to the (presumably russian) shadow brokers -- maybe he was paid, maybe he was hacked -- but it doesn't seem like a staggering leap to make.
certain politicians aside, exposing TS networks to the internet is not only difficult inside an organization that 'lives in the dark', it will get you severely reprimanded if not fired from your very well paying job, and possibly scuttle your ability to remain cleared and continue your career. classified networks are designed from above so that as much as possible this isn't an issue, and i'm certain it's only intensified since snowden.
i'm not saying by any means it's an impossibility that they were hacked, but i don't see any reason at all to favor that conclusion. like i said, martin worked for TAO, the source of everything the shadow brokers have released. why would you come to the conclusion that they were hacked vs an already known, very likely source candidate?