The reason is simple,. Obamacare. Big businesses are using more part timers so they don't have to do health insurance for them, also, under 30 hours is most states they get no vacation or other perks full timers get. Note that the start of the unemployment rise was about the same time Obamacare got voted in. 2010, just after the 2008/9 major recession. Jobs were never the same after that.

http://www.shadowstats.com/alternate_data/une...ent-charts

http://dailycaller.com/2015/05/15/just-14-per...s-waiting/

http://www.epi.org/publication/class-of-2016/

http://www.newsweek.com/2015/06/05/millennial...35821.html