The direct cause in this case was the PF reacting subconsciously to one piece of information, which was seeing a big and increasing piece of blue of the PFD, no doubt the result of many hours of training and conditioning to follow the ADI regardless any sensory illusions.
Given that it was middle on night the PF may not have been fully aware of how fast the display had changed.
Even if fully awake he may have had the sensation of having nodded off for a bit so would be less likely to suspect a technical fault.
Anyone who has ever had a close call (or worse) with falling asleep while driving knows the feeling...