This physician has given a couple of lectures at conferences of ARCE (American Research Center in Egypt). Sheldrick was the one, I believe, who first proposed that Tut was killed by a kick to the chest by a horse. He subsequently revised his hypothesis to death by massive chest trauma caused by a hippo bite.
Neither hypothesis is supported by evidence. Some of us involved with ARCE have wondered how this guy got to present his hypotheses not once but twice. An Egyptologist I know suggested he probably knows someone high up on the national board of the organization. I have not heard this fellow's lectures nor have I met him. Evidently he's a very nice person. It's just that these hypotheses are rather...oddball.
It all stems from the unusual fact that the body of King Tut is missing its sternum and several portions of ribs. The image below is from the first X-rays taken of Tutankhamun's mummy, in 1968:
There are two theories behind this. First is that the embalmers did it during the mummification 3,300 years ago, and second is that it was done to the mummy during World War II. Why the embalmers might have done it is not clear, unless there was trauma to the sternum and the embalmers felt it necessary to remove it. What might have caused the trauma in this scenario is unknown, but Sheldrick's hypotheses do not fit the extant evidence. A fatal kick from a horse would've caused much more extensive damage to the rib cage, which is perhaps one of the reasons Sheldrick abandoned this scenario. Hippopotami have one of the strongest jaws in the animal kingdom and a fatal bite to the chest would've caused most if not all of the rib cage to implode and collapse—and that is definitely not in evidence.
The sternum was physically removed from the corpse, as were the portions of ribs connecting to the sternum. All of this was clearly carefully sawed out.
If you look closely at Harry Burton's photos of the autopsy of Tut's mummy (1924-25, I believe), you will see garlands of flowers arrayed across the corpse's chest. The chest appears to be intact at that time, meaning it would've been intact 3,300 years ago when the boy-king died. Proponents of the perimortem or post-mortem removal of the sternum (meaning, it was done 3,300 years ago) note that perhaps the embalmers placed the necklaces of flowers on the chest to conceal the removal of the sternum. This is not a likely explanation because many mummies excavated in situ have been found with flower necklaces on their chests.
The same proponents note the clean cuts to the ends of the ribs and state the sawing of ancient bones would leave them splintered at the tips, thus the sawing must have been done by the original embalmers. I also don't see this as definitive because certain types of saws can perform this procedure on ancient bones without splintering them. Such saws are often used in forensic sciences.
This brings us to the second theory, involving World War II. With such unrest throughout much of the world, many Egyptian historical sites were left unprotected. Remember Harry Burton's photos which seem to show the corpse's chest intact, and covered by garlands. These were covered with so much ancient resins and unguents that Howard Carter had favored leaving them in place, rather than risking further damage to the body (his team had not been kind to the corpse when removing it from the innermost coffin). The flower garlands, then, were still in place by the early 1940s. The theory goes that modern tomb robbers entered Tut's tomb during the period of World War II and carefully cut out the chest to steal the flower necklaces. When the body was next seen for its first X-rays in 1968, the chest was fully exposed and a gaping hole was evident.
Both of these theories have their strengths and weaknesses, and it's likely the real answer will never be known. But the gist of it is, Egyptologists and forensic anthropologists and other specialists don't buy Sheldrick's hypotheses because they are frankly unrealistic. The best the available evidence can tell us, Nebkheperure Tutankhamun died from sepsis resulting from a severe compound fracture to his left distal femur. It was not the sort of injury from which an individual could've survived in that time and place. No one knows how the injury was sustained, so a chariot accident is just one possible scenario (the wound is consistent with a hard fall from a chariot, as would be a crush injury to the sternum, for that matter).