Has to do with angles and perception. In order to get that back light 'glow' around a person, the light needs to be able to beat the front light, overhead light, and side light. If it isn't powerful enough to do this, you don't get a glow, maybe just some shadows on the front of the stage/in the audience. Note how generally, back light is closer to the target then front light and generally uses more fixtures to do what it does. When I get to the office, I will post some shots of a show I did to explain it more with a photo.
Another fun filled fact with regards to angles of lighting. If your audience is looking straight at a stage and not down on it, or down on it at a slight angle, your side light and back light can actually be hard edge focused and the audience can't tell. I do this a lot for better control when using lekos for side and back light.