Not really, per Bruce Hoffman (a Georgetown University professor that's an authority on the subject)

"Terrorism is ... violence - or, equally important, the threat of violence - used and directed in pursuit of, or in service of, a political aim."[1]

Certainly this is political, but I think it's a stretch to call releasing stolen tax returns violence.

[1] Hoffman, Bruce. _Inside Terrorism_. New York: Columbia University Press, 1998. Print.