As I heard from one elder when another elder brought up the overlapping generation change: "they had to change it because it was obviously wrong - they're [the 1914 generation] all dead"
They're just making it up as they go along - there was a lack of a "sense of urgency" on account of the 1995 and 2008 generation changes, so they cooked the numbers to work out a way to create a deadline that is much closer. I think they probably genuinely believe that God is guiding them, but most of the doctrinal changes are pragmatic - miscellaneous random changes to keep the r/f paying attention and bigger changes that are driven by the passage of time, changing laws, etc.