Typically a ground sensing mechanism (AKA weight on wheels switch) will prevent the thrust reversers from deploying in flight. Upon landing, when the switch senses that the aircraft's weight is on its wheels, it allows the reverser levers to unlock.
I'm not familiar with this particular situation, but it's not unheard of to have the ground sensing mechanism fail. This can be overridden to allow certain systems to continue to functions (the pressurization system, for instance).