My brother Brian was ahead of his time. He followed the path outlined in this Time Canada article during the 1990s. It's the path of the twixters "marooned" in their 20s, done with college but not ready to commit to the long dry road.
More recently, however, it seems to be the rule among my friend's children and pretty common among the recent grads I know of from our neighborhood college (Macalester). I don't think the article describes these people very well though; besides establishing "twixterhood" as an official phenom it doesn't provide much insight into why this is (supposedly) common now. Is it simply a wealth effect? Is it a gestalt reaction to an unpredictable world, a "Lady or the Tiger" paralysis?
I think there's something different happening, and I suspect that there's not one simple explanation. I think of these social transitions as being manifestations of a nonlinear (chaotic) system. (The murder rate is my favorite example of such a manifestation -- it's driven by demographics and employment but transitions are dramatic and affected by many interacting sub-drivers.) There are probably some primary contributors, but also many peripheral interactions that cause a sudden prevalence spike.
It will be interesting to see if the phenomena persists, or if it recedes as quickly as it came.