Nothing actually has to hit you in the back in order for that area to be injured. A common cause of back injury that doesn’t involve any kind of direct trauma is whiplash. If you are in a car accident or similar, then a vehicle coming to a sudden stop can cause your entire body to get shaken. This then causes a shockwave to travel down the spine, moving each vertebrae and rupturing/moving/irritating the discs nerves located between them.
Whiplash can also cause a number of other injuries to the spine and even to the neck and brain. For these reasons, it’s always important to have any back pain checked by a physician or a physiotherapist -even if the accident appears to be very minor and you don’t feel much pain.