I think you just hit the fatigue life of the aluminum nipples. Unlike steel, aluminum can reach its fatigue life from the accumulation of small repeated stresses. When the fatigue life is reached there is a sudden failure of the material.
Wikipedia explains it this way: "One important structural limitation of aluminium alloys is their fatigue strength. Unlike steels, aluminium alloys have no well-defined fatigue limit, meaning that fatigue failure will eventually occur under even very small cyclic loadings. This implies that engineers must assess these loads and design for a fixed life rather than an infinite life."
There have been some pretty dramatic instances of aluminum fatigue failures. Although not the only cause, aluminum fatigue failure contributed to incident in 1988 when the roof of an Aloha Airlines 737 peeled off in mid-flight, sucking a flight attendant out.
Your instance is a bit less dramatic, but it seems like the same thing. Brass alloy nipples should hold up better or at least not all fail at once.