WTWizard's reasons for growth seem logical, especially 2, 4, and 5.
Fear: Expect the fear to be a reason for growth through 2014 as we near the century mark of "Jesus' Invisable Reign." Expect a spike about 2012 with the Mayan Calendar's end in December 2012.
Fake time reports: Who here hasn't fudged their time slips? They've always been filled out on the honor system. The only way that would ever end is if Watchtower used score cards like in golf, where your partner fills out your score (time) and turns it in.
New Changes: With Watchtower, it is always a moving target. Lower the required hours for pioneer service to have larger numbers of pioneers. Wow, instant increase! Lower the time needed to report to as low as 15 minutes. Wow, more publishers!
One thing that surprises me, is why they keep publishing the number of Memorial Partakers. I am amazed they don't lie about that number and decrease it to 7000. I guess nobody but us notice that number.