In all the cases where people have suggested incapacitation, fumes, hypoxia etc in order to explain otherwise inexplicable flying, I don't think a single one has been found to involve such an event after the formal investigation.
The simpler explanation is that the human brain is fallible, gets overloaded, loses SA and never regains it. Remember that these are rare 1 in 10 million events, most pilots will never experience such a degree of mental "lockup" through their whole career.