shortest and non professional definition of the word ....dissociation....in my location is that its a normal reaction to a positive or negative trigger.

put that in the normal non dissociative disordered terms a person can witness something traumatic ie violence through wars, domestic violence and other forms of violence and natural disasters and end up feeling numb, spaced out, disconnected from one self and their environment. (which is now why depression and PTSD contain dissociative symptoms and why people can have depersonalization\derealizaiton and dissociative amnesia problems)

that said in ...my location.....mental health demographics\statistics show that in order for someone to become DID and sometimes OSDD if the person has alternate personalities is if they have actually gone through extreme trauma before the age of 5.

DID and OSDD are the only dissociative disorders here in america that are so severe that dissociative type alternate personalities have been created, OSDD does not require having alters but there are diagnostic criteria for those that do with this disorder. most of the time in my location OSDD is used when a person doesnt fit other dissociative disorders and the treatment provider chooses to disclose why they dont fit in with other dissociative diagnosis's by designating which OSDD criteria that the situation better fits rather than other dissociative disorders in the list.

my point it can happen but because of the wording .....witness.... in your post if this was me and my treatment provider the dissociative symptoms due to witnessing violence would be called a PTSD symptoms in me not a dissociative disorder symptom in me. just the way diagnostics in my location differentiate between witnessing vs actually undergoing the abuse.

since this happened during your therapy session my suggestion is talk with your treatment provider. they will explain what happened and why and what its called in your location and what the treatment for it in your location is.