Sometimes the reason is difficult to be known unless there are interviews/official statements from the author. It might be due to author's health condition (very often), private matters, etc, but in the end, it's almost always due to the deadline of manga magazine. If they can't make it on time, then they either have to postpone until next edition, or send what they currently have.
There are steps that mangaka have to do to get a chapter done (see also What are the steps involved in making a professional manga?): storyboard, sketching, inking, finishing. While storyboard might take the longest time (according to that post), without it, there's nothing to be published. After that, they have to do sketching and inking, which also might take time depends on their skill (anecdotally, while watching live streams of mangaka, this usually takes the longest time). The finishing is usually not that critical and can be done quickly and easily.
Other examples of unfinished chapter being published:
Bastard!!: serialized in Weekly Shonen Jump. On chapter 51, the last 2 pages only consist of text inside panels due the deadline.
Cherry Gale Kin: serialized in Monthly Comic Comp On January 1991 edition, which is the last chapter, page 13 only consists of underline sketch, and there's no final tankoubon that included that chapter.
Left: Tankoubon vol. 1, page 5. Right: Monthly Comic Comp. ed. 1991-1, page 13
Spiral: The Bonds of Reasoning: serialized in Monthly Shonen GanGan. On November 2005 edition, which is the last chapter, some of the panels were only drawn in pencil sketch. However, they were fixed in the tankoubon version.
Reference: Unfinished chapter published in manga magazine (in Japanese)