It was part of a broader redesign that was intended to combat document fraud. The New York Times notes in New 'Green Card' To Tackle Fraud (August 24, 1989) that the color and other characteristics were to change, "largely...to stem a black market in alien certification." It reports that forged cards were thought to number in the tens of thousands.
New security measures included "a special blend of ink" used to give the card a pink color that was hoped to be "difficult to duplicate," as well as embossed lamination, optically variable ink, and machine-readable data on the back. Other security measures that were already in place included a photograph of the bearer along with the bearer's signature and fingerprint.