ClydeClydeson

This is not true and is likely based on a (regularly repeated) misunderstanding of fatigue limits.

1. Fatigue occurs when stress is applied cyclically; ie. low stress -> high stress -> low stress etc
2. Fatigue failure occurs when stress cycles introduce microscopic flaws that grow with repeated stress cycles and the flaws grow big enough that the part fails
3. There is such a thing as a 'fatigue limit' - cycles below a certain amount of stress that can theoretically be withstood by some materials (like stainless) infinitely without a fatigue failure
https://en.wikipedia.org/wiki/Fatigue_limit

HOWEVER

Just because a material has a fatigue limit does not mean that the stress cycles the material sees are necessarily below that limit.*

Furthermore,
the absolute stress is not the only factor in determining whether something might experience fatigue failure - the amplitude of the cycle, that is, the total change from lowest stress to highest stress is also a factor, even if the maximum stress (compression or tension) might be much lower than the stress required to deform or break the part, it can still cause fatigue failure.

And unless there were gouges in the broken spokes from the chain falling off the cassette, those spokes broke due to fatigue, and since no one spoke acts alone, we must assume the remaining spokes have seen similar stress cycles, and they should be replaced.


*this same misunderstanding is often applied to steel bikes, and people say steel bikes will last forever, but if you remember the days when steel was by far the main material for bike frames, those frames - especially very lightweight frames - would often break after several seasons (or less) of hard use - YES steel has a fatigue limit, but there is no guarantee that the stress the steel sees is below that limit, which it would have to be for the frame to last an infinite number of fatigue cycles. The lighter and thinner the tubes and lugs/joints the higher the stress the material would experience under any load.

And, yes, an evenly tensioned wheel is much less likely for this type of failure to happen, but OP's spokes have already started breaking, so likely weren't evenly tensioned, and so have been pushed up to (some past) the point of fatigue failure. If spokes were unevenly tensioned then the amplitude (size of the excursion from minimum tension to maximum tension) of the stress cycles was enough to cause failure. ANd if some of the spokes saw these stress cycles, it's extremely likely that all of the spokes saw these stress cycles. The microscopic flaws that resulted in the first few broken spokes should be assumed to be in every spoke because they all saw the same conditions that caused the first few to break.