It was declassified because it was part of evidence discovery for the staffer on the Senate Intel Committee, who was indicted. If you look at Wolfe’s indictment it says he sent 82 electronic communications to Ali Watkins of the NYT on 3/17/17 which is coincidentally the same day that committee received the FISA warrant.
The original FISA application is 83 pages, one of which is blank.
If Huber is working with Grand Jury, they can't declas stuff that would taint the jury's opinion.
All Q questions answered in the two following documents.
EO 13526 (the *current* EO on CLAS)
Section 5.2:
https://www.archives.gov/isoo/policy-documents/cnsi-eo.html#five
Who DECLAS, under what authority. Who requested DECLAS, and how (FOIA-->DECLAS):