From Halopedia, the Halo wiki
McKaskill had served on the prowler Apocalypso long enough for Melissa, the ship's artificial intelligence construct, to become protective of him. When the ship picked up the Deep-Space Artifact, McKaskill attempted send a message to Herzog about it. Melissa, however, recognized the act as treason and intercepted it to keep her crew member safe from capital punishment.
After the ship had crashed in Lunar orbit and Major Standish acquired the artifact, Standish attempted to eliminate every former crew member including McKaskill. Although the brakes of his Hog were cut, he managed to escape the Major's killing spree. He went dark after that, taking James James' black taxi across the country.