Myotonic Goats

These goats temporally stiffen up when they get startled or excited. Here is the clip of the farmers scaring them.
I don't understand how this type of reaction could have been advantageous in the scheme of evolution. How did these goats make it this far when they are literally paralyzed with fear? Maybe when the flock is attacked the goat with the longest lasting seizure ends up being the "sacrificial lamb". Strange breed.