My later guesses about the author aside, I think it's caused either by conformism, personal fetishism, or both; for this reason, I wouldn't call it an action born of panic--not if the authors are fine with what they're making. Many authors will deteriorate their own stories just to force either an industry standard or the author's fetish into them.
One example is Mori Marimo's
Risou no Kanojo: on his (not sure of this, as
Marimo is a female name, but there's no indication of the author's sex anywhere I can find) Twitter account, he recently gave previews of new chapters--chapters in which he introduces a short-haired, angry-faced, flat-chested rival. The MC is strongly attracted to the FMC Sara--and the author spares no expense at depicting her as supremely beautiful (both according to his standards and in general), with his doujins' omake sections detailing Sara's physical characteristics as his favorites--but the rival is there anyway, as if she could somehow compete with Sara (or her sister Kou, for that matter). It doesn't make any sense, but mangaka all do this sort of thing, so I'm guessing Mori thought he'd do the same.
Another example is in Mishima Yomu's
Mobuseka: there isn't anything in the story that causes the MC Leon to contravene his established character traits more than Marie--and indeed, the story seemingly warps to favor her. A "lolicon"--and even implicitly self-described as such in a recent omake of
Seventh, recently--he has even made an entire LN spinoff for Marie.
In the end, if there are enough people feeding such authors validation through positive commentary (and, indeed, it seems most Japanese are culturally averse to expressing criticism) despite such poor writing, they will not see a reason to change course.
