No, doesn't have to be anything underlying. Just a freak accident. Get hit in the heart at just the right moment and it can cause all sorts of problems.
Tangential story about freak injuries - A few years ago there was a CNN reporter, Miles O'Brien, who was on vacation and his suitcase fell over onto his arm. He got a bruise and didn't think anything of it. A few days later he had to have his arm amputated. Turned out the injury caused acute compartment syndrome and cut off circulation. Not caused by underlying condition- it's just bad luck and could happen to any of us, anytime.