I understand what you mean by the "incest boom." I'm no expert on anything like this, but I'd guess that it's thanks to scanlators, pirating manga (well, anything really) becoming more popular, and more young Japanese people are trying to become mangakas. I'd also guess that Comiket and other events like it selling doujins or whatever of hentai and ecchi manga featuring incest or pseudo-incest (step siblings/parents, related by marriage, etc.) has been so popular so it's obviously a money-maker and will get serialized. I also think one of the big allures is that Japanese usually keep to themselves and the only people they talk to regularly are their family. This is all just a guess though.
I also remember like 20 years ago thinking that I'd marry my sister when I was like 5. Not into incest, either. 😔

Wincest sells.
Though I don't really know about booms.
Some undertones as niches might appear more often, but
explicit stuff hasn't really gotten anywhere.
Not sure why, but I guess it's kinky enough to spice things up
while not reaching the what the hell is going on level, such
as smexy time with eels or vomit swapping.
That stuff would also appear more often if people wanted to see that stuff.
Things always start out as Jesus Christ how horrifying, but eventually
you just start to like the stuff, same goes for futa and NTR, though
I think they're a bit harder to approach, next level stuff when it comes to debauchery,
as they are not only differing of the normal Hollywood cowboy vanilla stuff
people are used to see, but also cause feelings that often aren't part of the experience,
still easier than bestiality and scat though, which not only are Jesus Christ how horrifying
to some extent, but also nearing the point of I find this hard to fap to, as
the point prolapses from erotic sensuality towards intangible manifestations
of human creativity or lack there of.
I recall a post on one h-site that wincest is the most searched tag
and the stuff has approximately over twice the hits of the rest or something
like that, been a few years ever since then, but I remember it because
that was when the admin of the site went from hating the stuff to getting used to it.