Automatic tailgate door, which was open for loading, randomly came down and struck me in the head. Door did not retract on contact but continued downward. No one was in vehicle. Neither keyfob or button were touched. I'm on blood thinners, this was a dangerous head injury. Could be extremely dangerous for children. This is second time this happened, but previously the door didnt hit me, I jumped back. This time, I was leaning in to load items. Again, I was just placing items in back. I was alone. Purse was in cart. There was no way any button on car or fob was pushed to close door. It closed on its own. Very dangerous!