It wasn't somebody who was winning; it was Horace Greely, who ran against U.S. Grant in 1872. He'd lost the election in a landslide, and then afterward he died. His 66 electors became unbound. 3 of them went ahead and voted for him symbolically, and the rest went ahead and voted Grant. Since Grant already had sufficient electoral votes to win, this didn't make a difference to the election; it just means that if you look at the electoral college totals without knowing about Greely, it makes Grant seem way more popular than he was.