Yeah I think it’s especially construction by analogy with similar words (phonologically or semantically), people tend to say words in a way similar to other words when their mind sees a possible pattern, e.g. if you know it’s mug->mugs, hug->hugs, rug->rugs, pug->pugs, tug->tugs, nug->nugs, you think “obviously it’s wug->wugs” for -/ʌɡ/ words, especially monosyllabic ones, but also maybe polysyllabic words or words that sound similar in some way but not the same, like -/ɔɡ/, -/ʌk/, -/gʌ/, etc. This also goes for words with somewhat different phonologies but similar semantics, e.g. if you know child(er)->children and broth(er)-> brethren, you’ll probably think it would look something like sister->sistren (which is a less common dialectal variant actually). If you know goose->geese, foot->feet, tooth->teeth, you’ll probably think it’s moose->meese and noose->neece and shoop<-sheep and hoof->heef unless you have a reason to expect irregularity. Or mouse->mice and louse->lice, you’ll probably think house->hice and spouse<-spice and blouse->blice.

But if you haven’t processed enough words that pluralize in a way other than just appending /s/~/(ə)z/ to the end, you’ll of course just think “gooses” and “tooths” and “fishes” and “foots” and stuff. Like what children do. Also common for children to say is “fishies” and “goosies” and anything else with /iz/ added at the end, since singular /i/ and plural /iz/ are common for adults to use as a diminuative/cutesy way of saying them, and the kids pick it up of course.

All these sound cursed, so I’d rather not think about it too much.