The descend rate was not constant – they INCREASED it most likely as a result of observing the falling slope of the ravine on Radio Altimeter.
I don't think they did it as a reaction to the RA. I don't think they did it at all - the pitch setting remained the same, and the AT kept the engines at idle. It was just a natural effect of the airplane's gradually losing the airspeed - w/o any involvement from the crew.