Cockpit crew are supposed to have quick-donning masks so that they can quickly get oxygen in the event of a rapid decompression. With that said, a sudden loss of pressure is usually a very violent and rattling occurrence so I suppose it's possible that they could have been hypoxic rather quickly, especially at FL380. At that altitude the would have approximately 20-25 seconds of useful consciousness before passing out.

The AP would not enter a rapid descent on its own.