Probably impossible to determine exact cause from the article. I looked up that pass and it has 28° and is closed to vehicles over 7.5 tons for that reason.

However, the officers quoted probably are right to not suspect actual brake failure. with a long grade like that normal brakes seem to be overburdened due to heat fading. Assume some cheap pads, not perfectly adjusted brakes etc. and you see how that can happen. A long 28° decline would call for 180+ mm front rotors and good hydraulic brakes IMHO... don't think most bikes have that.

Obviously we will never know if the girls made a mistake. But with the prevalent bad practice to declare rear brakes the main brakes, and not teach people how to brake properly and scare them off the front brake.... i can see how she may not have used the front brake as intended. Hence her thinking the brakes failed. But this is just speculation....

Sad story....