Answer by taking a quote from the following article:

William Henry "Lone Star" Dietz (August 17, 1884 - July 20, 1964) was an American football player and coach. He served as the head football coach at Washington State University (1915-1917), Purdue University (1921), Louisiana Tech University (1922-1923), University of Wyoming (1924-1926), Haskell Institute--now Haskell Indian Nations University (1929-1932), and Albright College (1937-1942). From 1933 to 1934, Dietz was the head coach of the National Football League's Boston Redskins, where he tallied a mark of 11-11-2. He was inducted into the College Football Hall of Fame as a coach in 2012.

Dietz's Indian heritage was first contested in 1916 after former neighbors who settled on the Pacific Coast heard he was posing as an Indian. In December 1918 the Federal Bureau of Investigation looked into his heritage after he fraudulently registered for the draft as a "Non-Citizen Indian" with an allotment. The Bureau found he had taken on the identity of James One Star, an Oglala man of the Pine Ridge Indian Reservation 12 years his senior who had disappeared in Cuba in 1894. Dietz also claimed he was the head of an American film company that produced propaganda films for the war.  Dietz divorced De Cora in November 1918, charging her with abandonment. It is not clear how much she knew about his true identity. She died six days after his indictment.  Dietz was tried in Spokane, Washington in June 1919 for the first offense. One Star's sister, Sallie Eaglehorse, testified after seeing him for the first time at the trial that Dietz was definitely not her brother. Still, the judge instructed the jury to determine whether Dietz "believed" he was an Indian, not whether it was true. Despite that others had witnessed his birth in the summer of 1884 or had seen him the following day, Dietz's mother Leanna claimed he was the Indian son of her husband who had been switched a week or more after she had a stillbirth. Dietz's acting ability along with his mother's fallacious testimony (to protect him from prison) resulted in a hung jury, but Dietz was immediately re-indicted. The second trial resulted in a sentence of 30 days in the Spokane County Jail after he pleaded "no contest".

Why was it contested?
the Federal Bureau of Investigation looked into his heritage after he fraudulently registered for the draft as a "Non-Citizen Indian" with an allotment.