I don't remember any time spent in diapers nor did I ever need training pants, so I remember when I was a kid (probably like 4 or 5) I found some old diapers in my grandmas house and I took one to the bathroom and put it on myself; they were probably just some luvs or something, anyway I put them on because I wanted to know how it felt, because even at the time I didn't really know what it was like, and spent the whole day like that. Later when I was watching a movie with my grandparents they must have seen it or heard it or something because my grandma asked if I was wearing a diaper, and I lied and stuff but it was obvious, so she called my dad and I forget the details about what came of that but it was nothing too special. I know I had to take it off though, and that I didn't use it.
much later in life I had a few other encounters; once when I was at a public bathroom I found a diaper just sitting on the toilet paper holder and I put it on (don't do this obviously), I had a hard time with getting rid of it, but nobody ever found out. Then when my dad got a new girlfriend one of her kids was wetting the bed so she had pull-ups, and I took one and wore it... and one became many over time, but never enough to cause suspicion. While I was wearing my sister's pull-ups/goodnites to bed, I used them instead of a pad, and eventually I tried wetting them too, and after wetting them is when I finally decided to see if this was a normal thing to do, and one google search later I knew it wasn't. so as a teen I read stories from abdl homestead and I think it all kind of grew from there.
I know this was a bit long, but I don't think it's one experience that triggered it for me, it was more just a summation of genuine curiosity. and I think subliminally it was probably always meant to be.