Eliezer's father was almost thrown off of the train because he looked dead.
After months of abuse and deprivation at Auschwitz, the prisoners learned that they were to be deported to the center of Germany because the Russian front was approaching. The starving prisoners were loaded into "an infinitely long train, composed of cattle wagons, with no roofs...(they were) pushed...in, a hundred to a carriage, (they) were so thin". In the packed wagon, Eliezer's father huddled, wrapped in his blanket, wasted and inanimate (Chapter 6).
At intervals the train stopped, and the prisoners were ordered to throw out the dead. Desperate and deprived past caring, they were maniacal, eagerly tossing out bodies so that there would be more room for the living, stripping the corpses first and fighting over their thin scraps of clothing. At one of the stops, two men came up to Elieazer's father, thinking he was dead . Just as they were about to toss him out, Eleazer threw himself on his father's body, shouting and slapping him and rubbing his hands, trying to rouse him. As the men attempted to pull Eleazer away, his father's eyelids moved slightly and it was evident that he was still breathing weakly. Disappointed that Eleazer's father was not yet dead, the two men moved away (Chapter 7).
The other answer is correct in that it is Elie's father that is mistakenly thought dead, but I must say that the rest of the passage overstates the moment. One time, not "at intervals" the train stops for the prisoners to throw out the corpses; because "they would have more room," the "living were glad," but no where does the text suggests they "were maniacal" or "eagerly throwing out bodies." Also, the prisoners "undressed the bodies"--they were not "stripping off clothing" and nowhere does the test mention anything about "fighting over their thin scraps of clothing"--in fact, the text shows them to have the opposite mindset: "eagerly shared his garments." And lastly, the text does say "the two men went away," but not that the men were "disappointed that Eleazer's father was not yet dead"--this qualification is not stated.
The words matter. In fact, Wiesel notes that his concern "did I use the right words?" stays with him (xv).
Wiesel, Elie. Night. Trans. Marion Wiesel. New York: Hill, 2006.