Anal area - sign of any allergy such as inhalent or food. Food is rarer.
Look next for muzzle area and ears, and the feet.
Not so much the butt but the lower back, and one does also lick his feet.
Followup.... The one who licks his back and feet also is allergic to plastic, His face breaks out if we use plastic bowls, so we now only use stainless steel.