I have nothing specific on the cause of the damage, other than having seen many other rifles damaged by bits of shrapnel. The forward section of the damaged area shows clear small shrapnel damage (vice being struck by shovel or dropped on a rock). The rear area shows clear knife-cut marks where the splintered piece was cut off. My photo doesn't show it well, but the damage is not as exaggerated as it looks... meaning, you don't feel the missing wood so much when handling it.
But yes, it is just my own opinion.
Nothing worthwhile ever comes easily.