re: What the heck happened?

Unless you could get away with pouring the second while the first was [very slightly] tacky, it would seem sanding, for adhesion, would be a must, just like with poly.

Sounds like you’re right. It wasn’t a matter of air transferring from the wood into the pour, since the first was already dry and, rather, a matter of the second layer breaking loose from the first.

For my old 50-50 resin mixes, which I bought in two five gallon containers at a time, sealing the wood was critical for the initial pour, to stop air from escaping the wood into the resin. After the pour, I’d flash it with a torch, which drew the few bubbles that remained out. Obviously, many finishes can’t be done that way, least you find yourself on the other side of a wall, without having gone through a door.

I should note I had this problem with SOME pours when I poured over brass on wood, but only some times. It may have been a temperature difference too. The resin lifted around the edges of the brass, where it met the wood, but not off the main surface.