There was a time two and a half years ago when I was really getting into ABDL, wearing Abenas nonstop for weeks, and only using the toilet for #2. At some point, I totalled my car, broke up with my very close mate, and had my unpleasant older brother move in with me, all within a one-week span. It turned out that I started waking up wet without control, and I excitedly tried to isolate and replicate the causes. Common factors for a wet night, as I discovered, included going to bed with an empty bladder and a stomach full of juice, diapered and already in a "baby" mindset. It did not occur to me that it was most likely caused by the stress of recent events, as many experts say.
For the past several months my closest real-life friends have been jerks to me. They've proven unreliable and insensitive, and more buddies than real friends; for example, if we all had the idea to play Halo, we'd play Halo together, but if I happened to be in my apartment nobody would actually tell me they were playing. As a matter of fact I'm really quite excluded from the group as of late. Moreover they think that my problems are my problems, and incorrectly believe that you shouldn't have to tell your friends about them.
My apartment was a wreck. On a messiness scale, with 10 being "very difficult to walk around in", it was a 7. One day I woke up and immediately started cleaning, and didn't stop for seven hours straight until everything was put away and organized, dishes done, laundry done, trash disposed of, carpet freshly vacuumed, and even the bathroom mopped. For the majority of the time I was thinking unnaturally negative thoughts, complaining, insulting, and ranting silently at figments of my friends. It's uncharacteristic of me, since usually I'm a hopeless optimist and quite cheerful regardless of the circumstances.
That night I wet the bed, despite having not worn a diaper for half a year, and having not wet the bed in 2.5 years. It was not a particularly babyish night for me, and in fact pretty normal, aside from the insane amount of cleaning and constant spewing of negative thoughts during the day; I did have a full glass of water right before sleeping, but I've done that many times in the past with no such results. (Side note, my bladder was only half empty when I woke up.) I didn't wet the bed last night, and I probably won't tonight either.
Not much more to say about it. I'm not shaken up about it in the slightest, though I am taking it as a signal from my body that I should keep up my philosophy of not worrying in little circles.
tl;dr - The biggest causes of bedwetting are drinking before bed and significant stress in your life.