By voluntarily, I mean they weren't forced into the job market. They weren't fired. They chose to leave their head coach positions to be assistants at Notre Dame. Whether they made their decisions for personal reasons or because they saw a stint at Notre Dame to be a stepping stone to something bigger remains to be seen, but the former seems more likely because the jobs they had could have been stepping stones on their own merits.
Owens improved the Northern Illinois program for four years and then had a disappointing fifth season (10-19/4-12). While at NIU, she got to be head coach of the United States’ U18 and U19 teams during two of her offseasons. She was at her alma mater, and she's in its athletics hall of fame. It seems like getting Northern back to winning seasons would have been a better career move than walking away from the job to be an assistant if she were looking to move to a higher profile program. Owens is a great assistant coach. From the outside looking in, walking away from the lead job at her alma mater to rejoin Muffet's staff looks like a move that makes her happier more than it makes her upwardly mobile.
Cunningham was more successful at VCU, the most successful coach the program had. There are plenty of personal reasons for her to leave VCU to be an assistant - young kids and/or getting back to the Midwest and closer to family for example. It's also possible that she sees her current spot as an opportunity to move up to become ND's head coach when Muffet eventually retires. By then her kids will be a little older, she'll have her credentials as a head coach from her VCU experience, and her advancement could make the transition seamless. There will be other former McGraw assistants who will be great candidates, too (Washington, McGuff, Tsipis, and maybe Ivey by then), so Cunningham moving up is far from a sure thing; but career aspirations are discussed during interviews and that scenario must have been discussed if Beth intends to be a head coach again.
That's the long winded way to say that there is at least as much reason to believe Owens and Cunningham will be at ND for the long haul as there is to believe they'll be looking for other jobs soon.