As FBinNY alluded - in general - cyclic load is the cause.

However, there are several things that can contribute to that cause being detrimental sooner - OR - later.

1. Stress relief. When done as part of a build - even el-cheapo bargain spokes can go years without breakage.
2. Tension. Too little = excessive flex per cycle = earlier onset of fatigue. Adequate tension = less flex per cycle = later onset of fatigue. Quality spokes can hold out longer - but the inevitable will happen.
3. Spoke Diameter and Spoke Gauge mismatch. The closer they are - the less slop at the flange holes - the better the heads and elbows are supported.
4. Flange Thickness and Spoke Elbow mismatch. 6.2mm elbows are intended for today's 3.2mm thick alloy flanges. Put those in some classic quality steel IGH hubs from the old days with very thin flanges - you get unsupported elbows. Hence why folks will add spoke head washers to force the elbow and initial shank run up against the flange.

The biggest cause...

5. A wheel builder who doesn't pay attention to multiple variables all at once when planning the build - such as the variables noted above.

Those are the technicals...FBinNY and others have already provided the environmental causes - both in nature and with the rider.

=8-)