Possible stroke or fit?
I recall working with a guy who was very animated when we had lunch together, and then almost monosyllabic, but functioning not long after, making me wonder if I'd upset him in some way as he didn't respond when I spoke to him, just stared out of the window. Half an hour later, He then collapsed, fitted and was hospitalised, woke up 2 days later but sadly diagnosed with terminal brain cancer. Could it possibly be that something similar happened to the co pilot , explaining the change in his conversation ( to laconic) and he 'mis functioned'?