While helping with environmental survey work at Blue Lake a couple of summers ago, this alder branch broke along where we had cut a trail through an avalanche path brush field adjacent to Blue Lake Creek. I am not sure what created the bore hole, but my guess is a beetle of some sort. It seems likely that the branch broke where it did because it was weakened by the hole. I may be that the hole provided an opportunity for decomposing fungi to get started, which would have further weakened that location. It’s probably not easy to get the identity of the borer based only on these excavations, so I suspect this will remain a mystery for quite some time.
Update: confirmed it was a beetle, even narrowed it down a bit from comments to the bugguide post I made with this photo