Beat this theory up, I'm am curious what you think...
The recent growth is helped by the most recent JW baby boom. JWs start having kids when an Armageddon "deadline" passes.
Post 1975
Post 1995 (no more generation limit). Those kids are now baptism age (12-13) and that crop will continue to help growth stats for the next few years.
The other major groups are immigrants and third world development.