Is there some sort of marketing reason for this trend?
This answer is based on general book publication, not to manga book (tankobon) specifically, but I believe this also applies to tankobon.
The main purpose of a cover art is to promote the product.
The art has a primarily commercial function, i.e., to promote the product it is displayed on, but can also have an aesthetic function, and may be artistically connected to the product, such as with art by the creator of the product. - Wikipedia
Seth Godin wrote a blog article about this, focusing on the physical form,
Is the purpose of the cover to sell books, to accurately describe what's in the book, or to tee up the reader so the book has maximum impact?
It's the third because if the book has maximum impact, then word of mouth is created, and word of mouth is what sells your product, not the cover.
Tactically, the cover sells the back cover, the back cover sells the flap and by then you've sold the book. [...]
On the other hand, this is what Guido Henkel thought on his blog, focusing more on the digital form,
So, let me ask you this question… what is the purpose of the cover?
The purpose of a book cover is not to perfectly illustrate the story down to the smallest detail or to showcase every aspect and facet of the plot. [...]
The sole purpose of a book cover is to help sell the book. A book cover is a selling tool! Nothing more, nothing less. It serves the purpose to attract eyeballs and then get those people intrigued enough to click on the cover thumbnail and learn more about the book, which, hopefully, will then result in a sale. If visitors on Amazon do not notice a book cover because it is easily overlooked and disappears among other covers, it serves absolutely no purpose and is actually detrimental to the author because uncounted potential sales are lost right there.
You always have to keep in mind that for the most part you are trying to sell books to people who are not familiar with you and who do not know the book or the story—at all. It is the cover that will hopefully draw them to it. It is the cover that will hopefully connect with them and intrigue them enough to find out more. [...]
While both had a contradicting view, neither mentioned that the purpose of a book cover is to illustrate the content of the book.
One Piece cover page was always/often not related to the content of the book (well just some of it) in the past. But now, the cover's always related to the current chapter. Why did they change that?
I haven't researched about cover art on serial books, but I have a hunch that on later volumes, the more important thing is to retain current readers. By this time, current readers already have an idea about the story. That's why by using the story plot as the cover, the author can draw more attention to the current readers, while hopefully still attracting new reader's curiosity (to buy earlier volumes).
Again, Guido Henkel mentioned this on his blog,
Only then will you be able to tap into new readers. Readers who are essential for growing your customer base not only for this book, but also for your next.
Anecdotally, unless I've already heard/known about the work (in which, I don't care about the cover at all and just buy the tankobon), I only look at the cover art (and the title, if browsing online), and if it's interesting enough, then I'd read the detail (online)/synopsis (back cover)/if possible, read some of the early chapters. By this time, I won't care if the cover art is related to the story/content or not, nor do I expect it.