I'm a pilot - a shit one compared to these guys and their ilk who can fly 737s nonetheless, but anyway.
Some systems will happily fly the aircraft into a stall (I realise that is not what happened here, I am just using this as an example) if the pilot does not intervene or uses said system incorrectly. It will even scream the words, "STALL! STALL! STALL!" etc into the headsets whilst doing so. Part of the training in my particular aircraft is to recognise this and disable the autopilot and recover if this occurs.
This sounds well and good, but if these fellows had thousands of hours (it would appear 6k for the captain and 5k for the FO) of experience and muscle memory then a new system, which may not even be able to be disabled, and possibly not have had an emergency checklist associated with it may (may being operative term) have contributed.