At one point during the Test, the officials, Darrell Hair and Billy Doctrove, noticed that the ball began to reverse-swing and decided to examine it. They ruled that the ball had been scuffed and awarded England five penalty runs.
After the tea interval, Pakistan refused to come out and the match was declared forfeited after a lengthy debate. The Pakistani team captain insisted that the ball was scuffed naturally during the course of play.
Pakistan's captain, Inzamam-ul-Haq, probably shouldn't have been asking Joe Niekro and Gaylord Perry for bowling tips.
The linked BBC article describes this incident in great detail and links to many other opinions about the incident.
To be precise, the forfeiture was not because of the scuffing accusation but because the Pakistan team objected to the penalty applied. Scuffing does not equal an automatic forfeit.