never heard that in 35 years of shooting, the fact that it happened to most and not all suggests other conjecture.
any fire armcan be affected by climatic change. its the reason for composite stock. and evn if humidity does not affect the stock teperture does.
my bet is that those who shot high exposed thier wood to humidity. specifically the lower for arm to humidity.
the fact that they began shooting straight was maybe compensation for shooting high.
you said most... those whom were not affected during the rain... how was thier shooting affected when it stopped.
LE's are very sensitive to humidity, more so the forarm which influences the accuracy of the weapon.
there is a very good reason the factory coated all wood surfaces with linseed oil. done properly and regularly it has several benifits.
those shooters who shot straight, did you ask them what they do different? and what did they say.
soak that wood in hot linseed oil. stock makers and wood workers been doing it for centuries.
I learned along time ago and again in the military not to leave your weapon exposed to the elements, not just rain either.... I wonder if theres any truth to that. Even those who shoot the AR platform exercise that rule. Na... it must be a myth.