President Barack Obama's signature health-care law could also be spurring employers to cut hours, since they will be required to make insurance coverage available to employees who work at least 30 hours. Obamacare's so-called "employer mandate" doesn't take effect until next year for large companies and is delayed until 2016 for small businesses with 50 to 99 full-time equivalent workers.

In the middle of last year leading up to the mandate's then-planned 2014 start date, there was "a big spike up in part-time work and then a big drop down when it was delayed," said Guy LeBas, managing director at Janney Montgomery Scott LLC in Philadelphia. "There's clearly some impact" though "it's really impossible to separate that out from many other issues."

Perhaps most crucially, skittish employers worried about the sustainability of the recovery may be hesitant to create full-time jobs, instead sticking with part-timers who represent a lesser commitment because they often don't receive benefits.

"The fact that we just don't have enough jobs for everyone who wants one now means that people are taking what's available," said Valerie Wilson, an economist at the Employment Policy Institute in Washington. "That elevated number of part- time for economic reasons means that wage growth will suffer."