It could be installation error but can it be thermal expansion and contraction?
Most gun stocks are made from wood that was dried considerably. Because of such I doubt themal expansion plays any role. I think they break during the installation process or just from normal use and forces. Considering the distance from the pins to the stocks fulcrum to the end on the buttstock and the amount of material surrounding the pins, i can suspect it wouldnt take much force to cause a crack.