User blog comment:The.Outsider/Desmond - First Trailer! (AC3)/@comment-4681429-20121019141701/@comment-4685497-20121019142028

Because the Animus was suffering from a major computer virus (Just kidding), nah, because the Animus entered a fail-safe mode, it got messed up and Desmond needed to acchieve the Synch Nexus. Therefore, the Animus messed up and Desmond's face was blended together into a mix of his own, Ezio's and Altaïr.