I'm always curious what trauma leads someone to being into fetishes like ABDL. No judgement, 'cause we're all weird about something, but there must be something bad in a person's history to lead to this
Depends on which one. Someone wanting to be the "mother" or "father" can often be because something in their life denied them the chance and they can't adopt for whatever reason. For example, criminal and mental health records can prevent you from adopting. It can also just be an extrapolation of the "mommy/daddy" fetish. Honestly the "mother/father" is probably the least weird aspect of it since it's really just a natural thing people are expected to do, with an unusual ("deviant") target. It's also the easiest to hide.
For the "baby" it can often be caused by being forced out of the "helpless stage" in life too fast. It can also be caused by having too much responsibility that makes the person long to feel powerless. And of course, it can be caused by good old fashioned daddy/mommy issues. There's a bit of an appeal to being "helpless" before your partner, and humans instinctively want what they can't have. A lot of adults long to be kids again, this mentality just takes it to an extreme.
But those are the simple explanations and can be a base explanation for acts like cuddling, nipple sucking, cooing, and singing lullabies. Things that even people not into ABDL can at least understand. "Headpats" are often a bit of an extension of this.
EXTREMES are where we get the adults messing themselves in their diapers. But it's really not functionally any different than the difference between someone liking rough sex and wanting hot wax dripped on their chest while getting whipped with a riding crop.
Honestly once you wrap your head around the idea that most fetishes are taking one "desirable" trait or action and exacerbating it, a lot of them start to become more understandable. Still weird, don't get me wrong, and while it's understandable to HAVE different predilections, it should be implicitly understood that not everyone shares them. That said a piracy site that specializes in porn games is exactly the place you *should* be seeing all the weird shit show up. So I really just find it amusing that people lose their goddamn minds over seeing things that are commonplace enough to end up in Yakuza video games.