The Afghan military was probably infiltrated by the Taliban for years and years

People are people and people are intelligent.

And the intelligent thing for the Taliban to have done was to have their people, people sympathetic to their cause, join the Afghan army these past years.

Free training, spies, sleeper agents, people waiting for the opportunity to undermine morale and take their units out of the fight at the climatic moment -- why wouldn't any resistance group anywhere want all those things?

This would explain the current sudden collapse and, honestly, have made it inevitable.

imho