Why did US oil prices fall to negative?

For the first time in history, US crude oil prices became negative. Prices of West Texas Intermediate (WTI) crude oil for May fell to -$37 per barrel on April 20. Why did this fall happen?

April 23, 2020 by Peoples Dispatch

For the first time in history, US crude oil prices became negative. Prices of West Texas Intermediate (WTI) crude oil for May fell to -$37 per barrel on Monday, April 20. Why did this fall happen?