Tut was not a perfectly healthy young man, and even if he had been healthy young people died all the time of accident or infectious disease.
There is no good reason to believe At or Horemheb wanted to get rid of Tut. Despite his health problems he was shaping into a classic New Kingdom pharaoh and his mentors may well have been proud of him. His death was disaster for his dynasty and for an Egypt destabilized by Akhenaten's religious reformation. Far from desiring it Aye and Horemheb were probably devestated and took the throne themselves Les out of ambition than because there was no male left with a legitimate claim and Ankhesenamun wasn't made of the same stuff as Hatshepsut.