Historians place the blame for the rebellion on Irene Kneedander, Head of the Body for Protection of Magical Species (Humanoid), whose interpretation of her job title had involved attacking any Sasquatch that ‘stepped out of line.’ The arrival in Washington of the Sasquatch necessitated mass Obliviations and extensive repairs to headquarters.
-- Pottermore: MACUSA
An uprising by the hairy giants of North America which caused the Magical Congress of the United States (MACUSA) to move operations from Washington to New York City (Pm).
The blame for this is generally placed on Irene Kneedander, Head of the Body for Protection of Magical Species (Humanoid) at the time, who attacked Sasquatches who “stepped out of line” (Pm:MACUSA).
Sasquatch: from the Salish Tribes of the Pacific Northwest sæsq'ec meaning "big hairy man"
Washington D.C. was built in the late 1700s and rebuilt after it was burned during the War of 1812. MACUSA headquarters, however, was moved from Baltimore to "Washington" for safety reasons during the American Revolution before 1777 (Pm). So while it is not explained on Pottermore, we can probably assume that they would move as far away from the war as possible, which makes the Pacific Northwest a good bet, even though Washington Territory did not exist yet. Also, the use of the Salish tribe's word "Sasquatch" more likely connects the Rebellion to Washington State, and the fact that MACUSA moved once again across the continent to New York.