Really? Is there any growth in the WTS? Kinda thought they were stuck at around 8 million for some time now.
The growth rate is slowing but they're still on track for an average (not peak) of over 8.2 million publishers by 2020.
Historically the Watchtower has had its greatest growth in the aftermath of financial/political difficulties: Early 70's, post 1945 and post 1918.
But that is generally a trend for all religions. In turmoil people tend to turn to religion.