I take it you have seen in movies a big cake that is wheeled into a room where a birthday party is going, and at the right moment a dancer, that has been inside the cake the whole time, jumps out of it and starts dancing to the amazement of the gathered people. So, in conclusion, to jump out of the cake is to make a surprise appearance that noone has expected. A killed person can't reappear out of the blue (jump out of the cake), because people don't come back from the dead.
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