Endogenic
Because the term "endogenic" technically refers to any cause of a system that isn't strictly trauma, there are a variety of more specific causes for our system even if we are fully endogenic.
While we believe a system can be endogenic in the sense that they don't have a reason they're a system, we don't feel our system is that way. Because there could be multiple endogenic causes for our system, this page is mostly going to be about the general premise that our system comes from something other than our childhood trauma.
While we do have childhood trauma, we also have a childhood anecdote that comes from before our trauma would have become as severe as it later became. It is certainly possible for a system to form due to "mild" trauma, but I also feel that the general trauma associated with living with an alcoholic angry father would not have been what caused an intensely amnesiac and dissociative system that early on without any of the later bad experiences. If someone else had my experience and felt that caused their system, I would believe them, but I also just don't feel that that's how that trauma impacted me initially. We therefore usually believe we are partly endogenic.
As for what the incident was that makes us think we had a system without trauma, some context is required.
We were assigned female at birth, and Blue - the core self who is most connected to ourself as a child and to our body - is a lesbian, and they had some awareness of their desire to marry a girl someday from a very early age - something like 3 years old.
We have always been very imaginative, but we did not have imaginary friends until we were an older child - more like 8 - and we had to imagine them on purpose. The closest thing we had were some characters we made up who we genuinely didn't understand weren't real, which, in hindsight, was likely connected to our plurality.
However, earlier than that - when we were 3, which is how we know we were a lesbian that early - we were spending time with our adoptive mother and said, quote, "Someday, I'm going to marry a girl named Liz."
There was no girl we knew named Liz. There was Liz the Lizard from Magic School Bus, but we knew she was fictional, and we don't remember having a crush on her. If anything, we would have had a crush on Ms. Frizzle. We can't think of any reason we would have been so specific about the name, nor why we were definite we were going to marry this Liz. Not "I want to" but "I'm going to".
Sometime after this, we had an incident in which we were convinced we were Liz the Lizard in real life. We called our adoptive mother and father "Mrs. and Mr. Frizzle", and therefore everybody knew we were having some kind of a disconnect from reality. When we refer to ideas that our adoptive father might have known we had a system and exploited this fact, this is why we think he might have known that. While it could have been a psychotic break, it was also very likely a system.
While it is possible that this Liz did not exist in the system prior to the abuse we likely later experienced as a result of saying what we said, it is also possible that they did. If so, Blue would have been referring to an in-system relationship or a crush on a headmate.
This tends to be the preferred explanation of the system, because it makes the most sense to us. Knowing us, we don't feel we would have said we were going to marry a specific person unless we had a person or even a character in mind. While we didn't have a crush on Liz from Magic School Bus, we do have in-system relationships involving fictives that are moreso about the headmate than about their source.
Therefore, we believe that we introjected Liz from Magic School Bus due to imagination, and we are positive that imagination is part of why our system is so complex. While imagination may not explain the amnesiac clusters, we feel it explains why our system is so complex with such a rich innerworld. Additionally, if our ideas about the spiritual causes of our system are correct, there could even be an explanation for our amnesia such that it did not directly come from being traumatized.