Catherine Rampell compares people unemployed for more than six months to people unemployed for under six months:
Perhaps those who were unemployed longer were less desirable job candidates to begin with, which would explain both why it took them so long to find work, and why, when they did found new work, the job was relatively crummy. Or perhaps people who have been job-hunting for a long time are more desperate to take any job that becomes available, so they end up in less attractive positions. Or maybe the gaping holes on their resumes start to look more and more suspicious to employers, so the job options become narrower and narrower.
Or perhaps those who’ve been out of work longer have become so embittered by the experience of unemployment that any new job they take will be viewed as a disappointment.