There is a little-known bylaw that if the President of MACUSA is in any way incapacitated/unable to fulfill their duty, that the Director for Magical Law Enforcement shall serve as President until the situation is resolved.

The international magical community is Not Happy with how President Picquery handled the Obscurus situation. (Particularly the Ministry of Magic; Grindelwald or not, MACUSA tried to execute a BRITISH CITIZEN).

Thus, Percival Graves - rescued from his imprisonment, fresh from his hospital stay - is informed that he is now Acting President as Picquery is under investigation.