Ok, my 2 cents. Was a bike mechanic and have built all my own wheels:
If it really did detension overnight then REALLY check your hub for cracks/pullouts/etc. Check the welds along the axle to make sure that the flanges haven’t suddenly come together.
Assuming you haven’t had a hub failure, and assuming the wheel is new, then I concur with the bad alignment theory. When you rode your wheel the spokes were overstressed and permanently bent into alignment. This relieved the stress and therefore detensioned your wheel.
If the nipples are already junk I recommend a complete overhaul of the build. Buy a new set of brass nipples, take the tire, tube and rim strip off, go around the rim in at least three passes reducing the tension on the spokes in small increments until all are slack, then replace each nipple. Clean the threads and use a good spoke prep compound. Don’t bother to tension the wheel at this point. Keep all the spokes somewhat slack.
Now, align the spokes. Bend them at the hub flange with your thumb until they point straight at the proper spot on the rim. Likewise, bend the spokes at the rim SLIGHTLY so that the nipple aligns with the hole a little better. It’s ok if the nipple is off a bit, the shoulder on the nipple and the gromet will take care of that misaligntment. Now it should look like a wheel with no tension in the spokes.
Next, increase the tension in the spokes gradually. Make repeated passes around the rim until there is SOME tension in the spokes. Now true the wheel. Get the spoke tensions even, then true it again. Add 1/2 turn on each spoke to raise the tension. True the wheel again, even the spoke tensions, and true it again.
Keep doing this until the rim potato-chips, then back off 1/4 turn.
Just kidding… But you get the idea. Wheels are strongest when the tension in the spokes is enough to create immenent failure in the rim (potato chip). This is because riding on the wheel REDUCES the tension in the spokes.
The rim is pretty elastic, so only the few inches near the contact patch deflect. This deflection pushes in on the wheel, which reduces the tension in the bottom couple of spokes. The rest of the rim and spokes are pretty much unaffected. This reduction in tension is proportional to your mass times whatever acceleration you have going on. In straight and level riding, this is one Gee, but on a big drop it’s going to be a lot more, perhaps 10 gees. (Which means that the spoke tension on the bottom few will be reduced by oh, say 1500 lbs or more. Enough to bend the cranks…)
If the wheel is going to hold together the lowest spokes need to be tensioned more than the higest expected loads. If they go completely slack then the nipple can spin and the wheel goes out of true. That means building in as much tension as possible during wheelbuilding.
Lateral loadsboth increase and decrease the spoke tension near the contact patch, depending on which side of the wheel the spoke is on. Since uni wheels see large lateral loads, it is not possible to tension the wheel to immenent failure as is the practice with bicycle wheels. Only very hard sprints and jerky climbs put significant lateral loads on bike wheels. In these cases I recommend slack off on the tension a little and increasing the number of spokes.
I’m sure this is why so many freestyle wheels are built with 48 spokes. I used to have a very hard sprint, so my track bike had a 48 spoke rear wheel. My 36 spoke road wheel failed every time I gave it a velodrome-hard sprint in a criterium, so I learned to back off a bit on the road. I should have built a 48 spoke rear wheel for crits but it was hard to find a road hub with that many holes that was cheap.
Recommend Jobst Brandt’s excellent wheelbuilding book.