It appears to have been refinished at some point.
It may have been oil soaked previous to that refinishing. The person doing the refinishing may have subjected it to a prolonged acetone bath to draw out the oil.
This is just speculation but that's the most likely scenario.
I don't have any personal experience with this but there's lots of warning to that effect that it should be avoided.

I doubt the humidity difference had any effect.
When humidity changes, the moisture content in the wood just gradually adjusts to the new environment.
The wood may have shrunk an almost imperceptible amount in Colorado. It didn't weaken it.
The oil and acetone is what weakened it.

Looking at the way it shattered like that made me think it's integrity had been compromised.
What is your gunsmith's opinion?
He should be able to tell.