John, if you lived where frost is a problem, I'd say the problem was caused by earth movement. But since you're down south, I'd guess you have critter problems. A foot deep is barely enough to avoid gophers, moles and ground squirrels; if there's a spot where the cable is a bit shallow then it would be vulnerable. Bury it deeper and you should be OK. You also want to make sure there are no rocks in the trench. Even if you don't have frost, vehicles driving over the cable can cause problems if the cable is in contact with a rock.
I don't believe you can use Romex underground even if it's in conduit. It's still a "wet" location, since any water that gets into the conduit won't be coming out. I sure wouldn't skimp on something I have to trench in.