Another possibility is that the already un-/underemployed are simply not looking as hard for jobs toward the end of the year because the relative emotional warm

Another possibility is that the already un-/underemployed are simply not looking as hard for jobs toward the end of the year because the relative emotional warmth and focus on family typical of the "holiday season" (Thanksgiving to New Years in the US) distracts them from their job search. It is also possible that white collar un-/underemployed aren't looking as hard during the holiday season because most offices are themselves so distracted by the holiday seasons. Anecdotally speaking, I've noticed a trend toward reduced hiring by companies during the the holidays. Finally, purely speculative but American companies may tend to avoid firing people during the holidays. Firing someone before Xmas is one of those tropes that appears in television dramas and comedies and is often performed by the "evil boss". This myth likely has some basis in fact.