> One possible interpretation is that the brakes when manually applied are insufficient to stop the vehicle when the counterweight provided by the cable is absent.

> Another is that the manual brake is insufficient, but the pneumatic/air (assuming those are the same) brake should be at maximum force, but for some reason, it wasn't applied at maximum force by either the brakeman or the automatic system.

Agree and I would add a third possibility:

The delay between detection and automatic full brake application, or by the cabin driver was long enough to allow enough speed/inertia to build, beyond the threshold until which the brakes would actually make it stop.