If you notice any of the other symptoms of
seizure activity immediately following the
behavior, even if it is the symptoms of a petit mal seizure, should be reported to your vet. If the dog displays no other symptoms of seizure activity either before or after the fly-catching episode, then chances are it is a doggie OCD, or obsessive-compulsive disorder.
Obsessive--compulsive disorders in dogs are not at all uncommon. Some of the will incessantly lick their paws.