Failure in venerable spoke at nipple: causes? 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.)