Hi Dave,
This is a very common problem. I find that the wider the board, the more it likes to warp. Not only are you making the wood thinner and easier to warp, but any time you cut a piece of wood, you release stress in the wood and movement can happen. Not to mention that your neighbor's shop probably has different conditions as far as heat and humidity.
Quarter sawn wood tends to not warp as badly as flat sawn, but also doesn't show the nice grains as well, so there's a trade-off there.
You can try adding some weight to the wood until it has time to re-adjust to it's new "home" and thickness, but it still may warp. Just part of the fun of working with wood.
