If the goal of the pardon was to handle a corrupt Judicial branch, then a better answer would have been for the Executive branch to nominate someone for a pardon to the Legislative and require a 2/3 majority of the Legislative to approve.
But something like that would fly in the face of the "Unitary Executive" insanity and would (I suspect) require a constitutional amendment, which is no longer remotely feasible.
A first comment said "without saying why". The second comment just says that this is blatantly not true, and that the rationale presented has been since confirmed as a very accurate prediction.
He feared Trump would force the DOJ to prosecute them.
And, based on what Trump's done so far with the DOJ and his enemies, he was right to do so.