This question of a 'trigger' is an interesting one. My interpretation is that before said 'trigger' there were no desires. some of the earliest memories I have center around my desire to wear diapers. I was spanked as a child, not abused. I was raised in a stable middle-class family. I am the oldest of two siblings. There was no diaper-discipline (only a threat when I was caught wearing one of my brothers pampers, I was 4-ish). Basically, my entire childhood is sprinkled with memories of searching for and trying on diapers that would fit me. Even the one series of events that would seem to have been my 'trigger' happened years after my first incident with diaper wearing. I would say that the desire to wear diapers beyond infant necessity, was ingrained as soon as I was potty trained (of which I have no memories, seriously, who does?)
So in the interest of this thread, I will indulge you with the experiences that allowed me to indulge in my fetish when I was a youngster. Here again, this happened many years after I was aware of my desires. I would not call this a 'trigger' but it was a defining moment.
Stop me if you have heard this one.
As a child, I spent a lot of time at my grandmother's house. She lived close by, and she had a swimming pool. During the summer I practically lived there. Just to drive the point home, I even began to speak with a New England accent (I'm from California, my grandmother is from New Hampshire). Anyway, My younger brother was a pain in the ass and my grandmother had a hard time with him, so typically I was visiting alone. Since my grandfather passed away when I was 9, I assume that the following occurred when I was at least 10. A typical (summer) day would involve me swimming from dawn to dusk. My aunt lived next door, so she and my grandmother would sit and talk while I swam. Sometimes there would be some neighbor kids that joined in, sometimes not. After a full day, I would come into the house and take a shower and eat dinner. One particular night, no one had planned for me to spend the night. My mom said it was ok and she would get me the next afternoon. Needless to say I didn't have a change of clothes. It was assumed that since I was there a lot, I must have left something to wear. Well not this night. My grandmother suggested that we launder my undies and after my shower I would just wrap a towel around and watch TV. So, here is where things get a little fuzzy. Prior to this I would spend a lot of time in the bathroom after my shower, putting on the towels as diapers. I had done this for months. I don't know if my grandmother knew this. However, during the night in question, someone suggested that instead of wrapping the towel around my waist, I wear it like a diaper. My grandmother found some safety pins and I went back in the bathroom and put on my terry-cloth diaper. I came out and she said I looked cute. I proceeded to run around all night in the diaper until bed time. Even though my undies were clean, I asked if I could sleep in the diaper. My grandmother, being the sport she was, agreed. Joy! I don't remember specifics about how well I slept or anything. I do know that I didn't wet the bed. The next morning, I woke up and changed into my swim trunks and swam until my mom picked me up. If this had happened once, I would have probably remembered it as just a great night. The thing is, whenever I spent the night with my grandmother (and my brother was not there) from that time on, I would ask if I could wear the towel diaper. I don't think this lasted too long. Not many memories have survived the years. Now as a kid I didn't think much of this, but I wonder if my grandmother told my mom. They didn't particularly like each other (paternal grandmother), so there probably wasn't much communication. My aunt knew, because she would typically be there when I was running around. I always felt like that was my little secret between her and me (and my aunt). I also remember being very particular about the fit of the diaper. I would fuss all night about how it looked. My only sadness is that there were no plastic pants to complete the scene.
I must reiterate, that prior to this, I was always on the prowl for diaper wearing opportunities. Obviously afterwards I was even more so. I do remember feeling free when I was wearing them. Comfortable. Much like I do now, even though it is in the privacy of my own home. I was too young and immature for any sexual feelings related to them. That came later.