This is such a "strange" crash that I am pretty sure we will have to wait for information from the black boxes (both of which have been recovered) to know what went wrong. The plane did backtrack and had the full length of the runway available for take-off. There are conflicting reports of just how much runway it used to get airborne, more-or-less certain that the slats and flaps were correctly deployed for take-off and remained that way for climbing. Initial climb was "ok" but then very quickly something happened leading to loss of thrust, plus it appears that the RAT deployed automatically. Plane appeared to hold correct attitude for climbing for the whole airborne part of the flight, but lost lift due to apparent loss of thrust... but why? Due to cockpit design, highly unlikely that controls for gear and flaps would have been confused and incorrectly operated. From current "reliable" information, very unlikely to be pilot error.
RIP to those who lost their lives and sincere condolences to the bereaved. Let's wait for the facts to fully emerge.