Hi Tiger,
You are right, there is nothing that YOU did wrong. However, whenever another person is in our company and we start talking about our feelings, then the other person has to have an emotional response to this. If you had told the story of the fight with your mother to a girl friend, then she probably would have joined in with comments about her and her mother.
Your boyfriends buttons were pushed by the experience. It has to do with how much stress a person can bear. Hurting one's back is physical stress not to mention the physical pain. Eyes that do not see, and ears that do not hear, heart that does not feel.
Since he saw and heard you, an emotional response was triggered inside of him. I would definitely talk to him and say you were sorry that he must have experienced some bad feelings and that you were not trying to scare him or hurt him. Who knows what past emotional hurt he has suffered in the past, now brought to light with the back pain.
You should not feel guilty but instead be more sensitive and understanding of what your boyfriend felt. Try not to probe, but tell him that if he ever wants to talk about this you are there to listen to him.