Tim % Fundin,
The effect of wood's MC on it's various strength properties is published in a very thorough book about wood science by a Finnish professor (in Finnish only). Wood's tension strength is at it's greatest at between 6-12%MC, but stiffness and compression strength hit their peak at 0% (actually, the author states that "most strength properties are at their greatest in 0%MC"). I translate this to mean that as wood gets drier and drier, even extremely so, a bow's draw weight and resistance to compression increase (and mass decreases, all good stuff), but the tension strength is "left behind" and fails, leading to a broken bow.
There's a ton of other interesting stuff I haven't found anywhere else in the aforementioned book, like the effect of continous freezing/thawing on wood's strength (not good), or how wood's basic S.G. (not just green S.G.) inside a living tree varies surprisingly much with the seasons, due to various substances in the green parts being sucked into /out of the wood.