Presumably bows break for similar reasons that valve springs or suspension springs in a car will break.
The constant cycle of stress leads to the weakest point giving up eventually. Of course not all springs break, so presumable the weakest point in the ones that break is weaker than that of those that don't break.
The weak point may be anything that concentrates the stress, a pin knot, a buried knot, a nick in the fibres. It may only be relatively weak because of a stiff point either side of it. It could be something several rings down inside the bow that happened to the tree years ago... a passing wizard leaned his staff against tree
or a squirrel gnawed the bark .. sorry getting silly now
OK metal fatigue take somewhat longer but steel springs exhibit the same properties in other respects too. E.G they take a set.
Del
PS... it's a quiet day here ... too wind for the flight shooting I was hoping for