@Squander despite what Adam said ... AB/DL is mostly a fetish. I have an academic interest in himself sexuality, and I've looked into a lot of the unusual proclivities people have. AB/DL does seem to have a small percentage for whom it's not sexual, but 90% seem have a erotic connections with the activities. There is an entire cottage pornographic industry specific to them, and sub culture with heavy crossover in the BDSM "scene". There does seem to be a percentage that came to as a result of actual incontinence, but that seems the exception much more than the rule. It is probably the case that it's a bit from column A and a bit from column B, and the urges for many people have mixed origins.
The truth is, nobody knows where fetishes come from, but many people seem to have them rooted in early childhood psychological events. It's no surprise then that diapers, and/or age play, would have a presence in that group of psychological anomalies.