My understanding is that one of the reasons caught up series can take longer is that while for existing volumes they can do a contract for multiple volumes, Japanese publishers are understandably reluctant to sign a contract for a volume that isn't published yet. This means that for such volumes JNC will have to wait until the next time they are in Japan doing a deal with the publisher to get the rights for it after it has been published (or at least when publication in Japan is set in stone). JNC then might have to wait for the digital files with all the text and art to be sent over.
I also assume that there is a stronger incentive for JNC to be prompt for series where Japan is several volumes ahead to maintain momentum on a series where them being prompt allows JNC to publish more volumes in a year, especially as it means less time for translators and editors having to re-read past volumes to help ensure consistency. *
This also presumably means that when you have translators and editors busy with series that have multiple volumes to translate the series with single volumes have a lower priority and have to wait for their original translator and editor to become free or a new one to be found. Some translators work for multiple publishers so their availability might not even be in JNC's control. The longer the series, the more work it is to bring a new translator or editor on board, so waiting several months for people to be free can make a lot of sense.
(* There might also be higher priority for series that JNC knows or suspects is just about to have an anime adaptation, but I can only remember one instance where this was obvious, which if I am not mistaken was Smartphone where they translated at least 3 volumes without a pause.)