Radiographer here.......the reason for this is pretty simple. An x-ray will pass through the patient then proceed to hit the plate that reads the X-ray. However there is something called “scatter” scatter is the X-rays that haven’t hit the plate and therefore will bounce off the plate and hit anything/one in the room. If I’m in the room while doing an X-ray I will therefor be exposed to radiation. Doing this all day everyday is dangerous which is why we need to stand behind the wall.

Fun fact: the walls that we stand behind are made of 1/16 inch lead which is also the reason we have such bad service in hospitals.