sounds like they used a non-flexible paint (or at least the primer). When that happens on a bumper which is flexible (fiberglass or Urethane), the bumper flexes at high speeds, and the paint doesn't. That makes cracks, and then the paint can flake off just from the force of the wind. They should repaint your bumper for free, and you should make sure they use an elastomeric paint for all three layers... primer, color, and clear. They should do three clear coats also.