Part of the problem with cross grain glue joints is that wood expands and contracts in response to humidity much more across it's width than it does longitudinally. Then you have that glue joint, usually an epoxy, which will inhibit moisture or humidity entering and expanding the wood from the end grain. And then you also have the relatively unsealed end grain of the extension under the buttplate. Many people think that most common stock finishes really seal the wood. But actually, they merely slow down the absorption of moisture. If that wasn't true, then we would never have a problem with wood movement. That thin epoxy glue joint isn't totally impervious to moisture, but it will allow a much different rate of moisture intrusion than normal end grain sealing. There is always more movement of moisture through the end grain because that is the normal flow of water and nutrients of the living tree.
