It's not a common figure of speech, but given the imagery I'd say it means: "put more problems onto somebody who already has a lot of problems", or maybe "make a problem worse"
John was having a terrible week - he lost his wallet on Monday, and he got the flu on Tuesday. Then, on Wednesday, his fiancee tossed another shovelful of dirt on John's coffin by breaking off their engagement.
Jane's car was terrible; it was 27 years old, it broke down every month, and it used lots of petrol. Yesterday, a truck driver tossed more dirt on her coffin by driving into her back-end when she was out shopping.