Moving away from my security threat speculation.
It could be something really prosaic like raging toothache, or laryngitis or something else which would have stopped him standing up to do a reading. Rather stupidly the RF announcement decided to state ‘personal reasons’ rather than a minor health problems to reduce more health-related speculation.
Whatever they had said would have caused speculation.
Isn’t that why we have a Royal Family to endlessly chat shit about their boring lives!