Fillers are usually the non-digestible, number-boosting add-ins, like corn, wheat, barley...
Dogs probably have an equal chance of becoming allergic to most things, but for some reason, they tend to be more allergic to grains than to meats. It could be because the foods these days are so loaded with grains and they're just more exposed to them, or it could be that they just have a natural sensitivity to them regardless of exposure.
All I know is I know a lot more dogs allergic to wheat and corn than to any meat.