I have a venerable (high spike count) rear wheel that has survived many a year before and after I purchased the bicycle.
Today a drive side spoke failed at the nipple while climbing. The road surface was smooth. I didn’t hit any bumps. I was seated but pushing power at cadence.
The spoke failed within the thread area.
Despite searching online I’ve not found a succinct explanation of the causes and actions of spoke failure at nipple (as opposed to hub).
If a spoke breaks, it'll go at one end or the other, because that's where the stresses are concentrated. Breaking at the nipple end isn't particularly unusual so I'm not sure there's any specific cause you should be looking for. (Both the spokes I've broken went at the nipple, for example.)
There is a general thought that the spoke/nipple interface is sensitive to tolerances.
If you screw in the nipple a bit too far when truing the wheel, then the un-tapped part of the spoke is pulled into the nipple, widening it causing stress.
Likewise, if the nipple ends up further out, so that there are a couple of threads not engaged, then that's a weak point in the spoke where it is thinner.
A perfect spoke/nipple alignment would have the nipple touching the unthreadded spoke firmly but not drawing the unthreadded spoke into the nipple.
tl;dr tolerances, spokes break, its just part of riding like punctures and wet days.
The good part is that replacing spokes is pretty straightforward, and any mechanically-inclined person can do so.
