I suspect that the stock had some internal stresses in the grain, and when it became so dry, the stresses just overcame the integrity of the wood fibers. I can think of no other explanation.

It showed no signs of cracking or splitting before the oil removal treatment. I clamped and glued it back with a cyanoacrylate and recut the checkering over it, but there is still an ugly dark line revealing the break.

SRH

May God bless America and those who defend her.