Plateauing: What has been the most occurring cause of weight plateauing?

Hovering around maintenance calories is common especially given the inaccuracies of caloric value's of most foods and peoples inability to measure properly. Being is a semi starvation state and depending on what your eating, is not a very desirable place for the body to exist and it fights back.

Has literally nothing changed in typical eating patterns (most fundamentally, calorie intake) or average total activity (not just exercise but also daily life stuff like job and home chores)?

If so, and if weight loss has been happening at a reasonably satisfying rate, then stops suddenly, high odds the explanation is water retention differences, not a change in fat loss rate. The answer in that case is patience.

This pattern may be especially common in women who have monthly cycles, since hormonal water retention can be surprising in some cases. (It's not common, but a few women here have reported only seeing a new low weight once a month, at a particular point in their cycle. More common is ups and downs that may look like stalls in weight loss.) There are other potential water retention triggers, though.

I'd strongly suggest reading this thread, especially the article linked in the first post:

https://community.myfitnesspal.com/en/discussion/10683010/the-weird-and-highly-annoying-world-of-scale-fluctuations/p1

However, if the first paragraph is true, and weight loss has slowly tapered off over many weeks then finally stopped, higher odds the person has found their current maintenance calories. A lighter body burns fewer calories every minute of the day than a heavier one, and that matters. Also, if someone uses extreme tactics - fasts weight loss rate, possibly sub-ideal nutrition - a contributing factor can be subtle fatigue that can bleed a surprising number of calories out of a person's daily calorie burn.

This last is not "starvation mode" in the sense of the body "holding onto fat". That's a myth. What I'm suggesting is better illustrated by research suggesting that fidgety people can burn perhaps a couple of hundred calories more per day than otherwise similar non-fidgety ones. That concept doesn't just apply to fidgeting, but potentially to almost any kind of movement: Think putting off high-effort home projects, cooking simpler meals, sitting more and standing less - things most of us might gradually change without even noticing. It's not enough to stop weight loss generally, but it can be a contributor via reduced calorie needs.

I think those are the most common, but of course not the only possibilities. What others above said is true, as well: There can be imprecision in logging, there can be psychological effects (such as deprivation-triggered overeating that the person doesn't log, mentally discounts because remembering feels icky, and out of step with how very hard they've been working to lose, among other issues).