There is or was an item in B757 QRH for loss of left hydraulics that stated: DO NOT USE AUTOBRAKES. Not saying that is the case here but if they did it can seriously impair directional control as A/B can only give symmetrical braking.. A B757 left the runway in LGW (Gatwick) many years ago as a direct result of this, Airtours I think. Basically with asymmetric reverse any yawing moment can be exacerbated by brakes on one side releasing due to increased lift on one wing and this just multiplies, especially if any crosswind (which appears not the case here) and also if for any reason speed brake not rapidly deployed.