If you follow enough authors long enough, some of them will turn to garbage.
Repetition. Gaping plot seams with stitches bursting.
Story twists that wring the neck of what was once great about the franchise.
Bad editing. No editing. Bad proofreading. No proofreading.
How could someone capable of such greatness now turn out this steaming garbage?
Very many people ask that question; few receive answers. In few cases will they ever learn which answers apply. In most cases, it will be one or more of the following:
Profitable Franchise Syndrome. Through careful promotion and at least some display of some form of talent at some point, a select few authors become cash cows. Any book with Cash Cow’s name on the cover is guaranteed X number of copies sold which will produce a profit of $ZZZ,000 at the minimum. Once an author reaches PFS, many publishers no longer give a damn what s/he writes. If the author also no longer gives a damn, there are no barriers to the publication of wretchedness. For obvious reasons, such a publisher will never sick a sharp-penned editor on said author, to tell the author what s/he needs to hear. No one will risk causing a bad case of mastitis in the cash cow, least of all the cow him/herself.
Writing is hard and ideaing is sometimes harder. Some writers loathe above all (even above “will you please read my ms?”) the question, “Where do you get your ideas?” When writers run out of original ideas, they recycle, or steal, or meander without aim. When writers get sick of sewing together mismatched plot points, correcting inconsistencies, and otherwise trying to reduce eventual scarring on the patient, they stop bothering. Why bother, when people will buy it anyway? The priority is not to write a great book this year. The priority is to get a damn book written this year, keep the milk flowing, keep the name on endcaps.
Dotage. Sometimes this combines with PFS. The Grand Old Author has written one more book, after years of not writing. How come these are so often like watching a former home run king hit .189 a year after he turns forty? Because the author is often, at this point, well washed up, and probably either wanted or needed money.
Freelancers. When PFS is reached, the writing may not even be that of the ‘author.’ I have seen books in which it was sadly clear that each chapter had been done by a different ‘lancer and no one had checked the stitching, resulting in a couple of dozen rehashes of the same background material as each ‘lancer felt obligated to make sure the reader knew the pertinent backstory. The author did not bother to remove these redundancies. This is how little they care.
Delusions of inherited writing talent. Just because Mom could write does not mean #2 Son can write. Just because Dad could write really, really does not mean #1 Son can write. This, however, may not stop the grown child of the Famous Author, because said grown child may be a career screwup who sees in Famous Author’s inherited glory and name One Last Chance For An Easy Living. By that time, Ma or Pa may be too old to care. He or she may feel that dues have been paid, and that if this will keep the kid out of debt hell, that is their business. And it is–except for the money that honest readers will waste on dishonest, substandard effort. That is their bad business.
Psychological rabbit holes. In fiction writing, and in particular with first-person fiction that is semi-autobiographical, at times the author reaches a Nietzschean moment. Remember his famous quote? “Battle not with monsters, lest ye become a monster, and if you gaze into the abyss, the abyss gazes also into you.” Life has taught me that Nietzsche knew what he was talking about, and writing life has countersunk all the screw holes for that lesson. Imagine you’ve got a major inner bugaboo (for example, sexual bondage and domination), and that it’s always been complicated for you. That balance of desire and aversion, the tension between two points, can be a productive wellspring for fiction ideas. It can also become the abyss into which you gaze too long, taking you to a place from which you can not easily come back. Your writing may reflect that, and it may get away from what your readers liked best about your work. Most anyone can take a mental illness (or some lesser condition/neurosis/syndrome/disorder/dysfunction/bugaboo) and use it to write a book. To do so without making the condition much worse, and without having the condition consume the franchise alive–that’s the tough part.
Fetishism. Closely related to the above, of course, and with some overlap, but not the same. One good example was a writer of Westerns with an obvious girlfight fetish. Rare was the book of his in which a pair of women didn’t mix it up. Do I even need to mention that blouses never survived a single battle? Okay, so let’s consider this. Suppose that’s your fetish, and while it does not consume you, you write enough of it to develop an audience–and business is good. Then you decide, okay, I’ve done my time on the pole and I can now de-emphasize my little kink. And sales fall off; reviews scowl. Your audience doesn’t like you so well when it doesn’t get its accustomed dose of the fetish. Tired of writing about breasts flopping loose in a fracas? You are stuck, because that’s the audience you attracted and that’s your brand. You may or may not be able to redefine your brand. You may just end up churning ’em out, supplying the expected unrestrained dairy tackle, because making house payments beats missing house payments, and writing what you know is easier than seeking new horizons.
There may be other reasons (opinions welcomed), but those are some of what I’ve seen. Your favorite author is like a car. It may run for a long time, it may be the best one you ever had, but one day you won’t be able to get parts for it. You may keep it as a classic, a project, but it won’t be your daily drive. You’ll get something you can rely on. By the time your heirs get the old one, they’ll wonder why you kept it so long past its prime.