I've seen a plug come out before, although not in the crazy way yours did. So I always carry one of these. Probably never need it, but it is small and light and does a good job. Of course if you hit it by a small rock it will pop it out. Crazy for that rock to hit it and turn it. I will never stop being amazed at the crazy things that happen when wheeling.
http://www.autozone.com/autozone/accessories/Needa-Parts-Universal-oil-drain-plug/_/N-25r9?itemIdentifier=256832_0_0_