I've seen it years ago. Did you apply it while it was raining outside or high humidity? Body filler hardens pretty quick but if there's moisture in the air or on the surface it will bubble. Or if the surface was cool or if freshly wet sanded and not totally dry it will do the same because when you mix filler with hardener it creates a chemical reaction that generates heat. Too thick of a layer of filler will do the same as a co-hort just told me, it's got to be applied in thin layers according to him. He rec's sanding the surface, wiping down with prep-sol or alcohol to absorb & evaporate the moisture then apply your filler.