Or perhaps they realized that the few high security document destruction machines they had on site, for daily sporadic use, wouldn't be enough to destroy the complete collection of documents, accumulated over the last 41 years that they've been present, in a timely manner?

According to DoD 5220.22-M, 5-705, incineration is probably the only realistic method in this particular situation.

The US has done the same in the past. When a foreign government kicks you out, you can't say politely ask "ok, can you give us a week to finish up here?". Or are you suggesting that they should have known they were going to be kicked out and destroyed everything in preporation?