Jamieson, Fausset and Brown on Haggai 2:21

21. to Zerubbabel--Perhaps Zerubbabel had asked as to the convulsions foretold (Hag 2:6, 7). This is the reply: The Jews had been led to fear that these convulsions would destroy their national existence. Zerubbabel, therefore, as their civil leader and representative is addressed, not Joshua, their religious leader. Messiah is the antitypical Zerubbabel, their national Representative and King, with whom God the Father makes the covenant wherein they, as identified with Him, are assured of safety in God's electing love (compare Hag 2:23, "will make thee as a signet"; "I have chosen thee").

shake ... heavens--(see on Hag 2:6, 7); violent political convulsions accompanied with physical prodigies (Mt 24:7, 29).