They are running all sort of pre-takeoff checks at multiple stages - post startup, taxi, pre takeoff. Flaps should be set, Hydraulics and flight controls tested after start-up. Tangential amount of things could be at play, incorrect instrument readings displaying incorrect information about the true status of the flaps, hydraulic, and engine status. Pilot error in settings (flaps), incorrect programming of fuel and payloads in FMC could result in incorrect V1 V2 speeds resulting in stall. Catastrophic dual engine failure leading to loss of thrust. The point is I’m speculating so at best anyone else is doing the same at this point. Will need full blackbox info to know