Only A linemen can BIB in the Free Blocking zone.
But, the defense can BIB in the back if they are going for the runner or a loose ball as long as it isn't a personal foul, pass interference, or illegal use of hands.
Rule of thumb I use is that if the defense BIBs near the runner, he is probably going for the runner, so no foul. If he does this and he is no where near the runner, then you have a personal foul for unnecessary roughness. So, I would never have a BIB on the defense.