I'm not sure about Afghanistan, but Ford let the Ambassadors make the call in both Phnom Penh and Saigon as to when to evacuate. It was Ambassador Dean in Cambodia who made the call for an early evacuation. Martin who was Ambassador to South Vietnam put off the evacuation until the last minute. The writing was on the wall for both, one went a bit early, the other I'd say almost too late.
But the decision was left strictly to the Ambassadors as to when to evacuate. They were the highest ranking official on the ground. The advancement of the Taliban was much more rapid than that of the Khmer Rouge or the NVA. The Khmer Rouge had Phnom Penh surrounded ever since August of 1973. The NVA started their advancement in Dec/Jan 1974/75. Phnom Penh fell 17 Apr and Saigon on 30 Apr 1975.
A week ago, no one ever thought the Taliban would be threatening Kabul this fast. One can say that was totally unexpected. But I don't know if the Ambassador was the one to make the evacuation call or if it was someone higher up for Afghanistan.
I'm no longer in or work for the military, so I get my news like everyone else with no insider information on it.