jimmuller 

According to Jobst Brandt spokes don't actually break from tension. In fact spokes can stand much more tension than the rim, and if you tightened them enough the rim or the nipple would break first. The main reason spokes break is fatigue from continued bending at the elbow. The entire spoke and especially the elbow is under elastic deformation. The load is reduced on each spoke when it hits the bottom of the circle and so the bend relaxes back closer to its fully bent shape. It then becomes re-stressed again when the spoke moves away from the road. This continued unbending and re-bending eventually fatigues the metal. The looser the wheel the more this happens. A swaged, i.e butted, spoke experiences less of this stress change at the elbow by having more elastic elongation spread along the thinner section. Thus it can go much longer before fatiguing to the breaking point. The "proof" of this is that spokes usually break when at the bottom of the circle where the tension is actually less rather than near the top where one would intuitively think the load would be greater (but actually isn't).

So when more than one spoke break on a wheel it might be an indication that all of the spokes have seen a lot of fatigue.