"Automatic" is probably not the best word to describe it because JW Elders still have to investigate the matter and determine what actually happened.
Voluntary disassociation was invented to avoid further prosecution under the Espionage Act. Interfering with military recruitment is illegal and it could easily be construed as interference for a religious organization to punish its members for joining the military.
When someone enlists in the military by signing the documents, raising their right hand and taking the oath, etc., there is no doubt that the act was voluntary and deliberate. The elders simply need to confirm that it actually happened.
Transfusion is far more complicated because there all sorts of things that could make the act either involuntary or only partially voluntary. What if they were unconscious? What if they were confused or scared or both? What if they were only semi-lucid? What if they didn't realize the consequences?
It's one thing if the person says, "Yes I accepted a transfusion. I was fully conscious and knew what I was doing. I'd do it again. I don't care if I'm one of Jehovah's Witnesses or not. Now get the hell out of my sight."
It's quite another thing if the person says, "Yes it happened, but I was very badly shaken up by the accident. I was a passenger in the car and didn't have my ID with me. I still want to be one of Jehovah's Witnesses and feel terrible about this. It would never have happened if I had been in full possession of my faculties. I really wish one of you had shown up to help me that night."