Human Ingenuity
We can speculate until the cows come home about what happened but - and pls excuse me for stating the obvious - the essential tennents of SAFE flying are :
(a) always (even in a LOE flight) have someone minding the shop > fly the aeroplane and/or stay in the loop when on auto > emphasize during the pre flight briefing that this is THE DEFAULT requirement overriding all others and especially a training agenda
(b) if the aeroplane is doing something (in AUTO) which one operating crew member does not understand or views as a hazard (less likely if you are following (a)) call out the problem and take manual control (disconnecting the AUTO function(s) as necessary to achieve control) and fly the machine into a stable condition (which usually means a GA when the a/c has passed the FAF). Don't be afraid of a GA even if your ego or the Co Regs for such an action weigh heavily on your psyche - the passengers and your loved ones will have reason to thank you for it
(c) we learn from our experiences. Whilst not disagreeing with those who advocate the development of "fool-proof" automation, no amount of fuzzy logic and clever software will eliminate or second guess the ability of human ingenuity to find a path through the Swiss cheese. So keep to SOPs (yes, including scanning the PFD FMAs) but maintain a healthy dose of scepticism and above all situational awarenes
(d) it does have help to have a bit of passion for this profession. When it becomes (or never was anything but) another job and your attention tends to drift whilst in the office (particulary below 10K) then it is time to shape up or maybe to look for a much more forgiving environment in which to earn your crust