One of the subpoenas was about PopeHat, who of course is public, and he wrote about it on his blog. Why demand twitter unmask a public account?
Speculated reasoning in the comments on the article was that it then gives an admissible account of who owns the twitter name. Otherwise the assertions about who owns it are inadmissable hearsay. That said it's still really weird that the accounts were subpeonaed
EDIT: fixed link. Just noticed it went to an old article. Apparently the new one is up but not listed on the homepage? (Similar topics so I didn't notice right away)
https://www.popehat.com/2017/10/24/in-which-my-identity-is-s...
Also, if I were trying to intimidate people who read a lot of Ken White's opinions, I wouldn't try to do it by filing questionable legal things about Ken White, thereby forcing him (as you say, a former USDA with a wide network) to enter into legal proceedings with me and inviting him to discuss my actions to his audience.
To a first approximation, that seems like it has a high likelihood of backfiring.