Boeing hasn't confirmed it yet, but everyone is pretty sure this is the issue. There is a new Angle of attack indicator on the 737 Max 8's and Max 9's. This was added due to the new engines on 737 Max 8, and 9 because the engines would cause the nose to raise, so the plane uses the new indicator to automatically level the nose. During takeoff, when the nose is extremely high, the plane believes it's about to stall, so it tries to level it's self out (this is what is believed to be the reason for the Lion Air and Ethiopian crash).