With the authenticity of the Shadow Brokers files being confirmed, the biggest question is: who leaked them? There's a small chance that it was a stupid accident in which an NSA hacker uploaded his whole toolkit to a non-secure server and someone found it there.
Somewhat more likely seems the option that they came from an insider, and in that case, this leak doesn't stand alone, but fits into a series of leaks in which, since October 2013, highly sensitive NSA data sets were published.
So almost unnoticed by the mainstream media and the general public, someone was piggybacking on the Snowden-revelations with leaks that were often more embarrassing for NSA than many reportings based upon the documents from Snowden.
Again, obtaining such documents through hacking into NSA servers seems less likely than the chance that someone from inside the agency took them. If that person was Edward Snowden, then probably someone with access to his documents could have started his own crusade against NSA.
If that person wasn't Snowden, then it's either another NSA employee who was disgruntled and frustrated, or a mole for a hostile foreign intelligence agency. For an individual without the protection of the public opinion like Snowden, it must be much harder and riskier to conduct these leaks than for a foreign state actor.
Former NSA counterintelligence officer John Schindler also thinks there could have been a (Russian) mole, as the agency has a rather bad track record in finding such spies. If this scenario is true, then it would be almost an even bigger scandal than that of the Snowden-leaks.