Why does acute glomerulonephritis usually decrease urine output?

Filtering unit clogs: This disease causes inflammation and clogging of the filtering units in the kidneys. With less filtration there will be less urine output.

Changes in function: One of the defining features of glomerulonephritis is an alteration in the kidney function. One of the functions of the kidneys is to help regulate fluid balance. So if the kidneys are affected by glomerulonephritis, fluid balance may be compromised.