In general, shoulder injury happens when the muscles and ligaments that hold the shoulder together are stretched beyond their
normal limits or when you perform
movements that put too much stress on the shoulder and cause tearing of the ligaments that hold the shoulder muscles attached to the bones.
Your adult cat may react aggressively, which is
normal, and you'll have to separate both the cat and the kitten and
limit their
movements so that they never meet each other until the kitten becomes several months old.