Namely, he uses the methods of the Left, and the Left hates him for it.  Read CBS News’s explainer on the issue at hand:

Presidents do have sweeping authority to declassify records, but there is a process that is normally followed.

Generally, a president’s instructions to declassify documents are first written down in a memo, typically drafted by White House lawyers, which the president would then sign. Relevant agencies are usually then consulted and when a final decision is made, the document would be marked, with its old classification level crossed out, and stamped, “Declassified on X date” by the agency in question.

“Generally,” there is a process “that is normally followed,” but legions of hostile journalists and talking-head lawyers cannot point to documentation of that rule.  (Presumably it exists even if they think it does!)

Should Trump have followed the traditional process?  Certainly, but progressives do this sort of thing all the time:  When it is convenient, they throw away the process that everybody had previously agreed existed in order to do what they want.

As much as I’d prefer adherence to expectations, the overarching principle that the rules have to apply equally is true on the higher plane, as well.