I kinda figure that the pole isn't secure in the ground, like whoever installed it did it poorly and not deep enough into the ground. When you back up into it, it begins to tip over until you have the top under the car. When you try going forward, it rights itself. I've seen something similar with different kinds of poles and when they are upright, they look completely fine.

I figure that if you backed up with enough force to drive up the pole, the bumper would be more damaged.