The atrophy might be related with the gradual stifening of the spine, the cervical area in this case.
Last years i have been dealing with a cervical hernia, and one of the sympthoms was the left triceps atrophy, caused by the pressure in the nerves.
Muscle and therefore skeleton imbalances might agravate this condition. One thing that most bodybuilders have is the shoulders rotated forward, pulling the neck forward, creating stress points in the cervical area.