I was thinking "Maybe the mangaka is over worked" which would explain the long delays, I could understand this then. But nope, as far as publicly available info is concerned, I can't find any evidence he's working on other things, at least in the novel/manga industry.
What is interesting is that ever since this manga started releasing, once a year there's a two part release. There's no date consistency with this two part release though. Usually a multi-part release is a consistent policy, often so that they can release content faster for a series(ex: weekly instead of monthly), but that's clearly not the case here as the parts are still monthly when they happen.
My guess would be that either the mangaka gets an annual vacation, or gets tied up with work in some other industry.