My 3 year old has had croup 8 times in the last 6 months. he also has asthma. how does asthma cause croup?

Puzzling pattern: A croupy cough is created when the rubbery airway of a kid 6m-6yr coughs hard enough.Many germs trigger it.Older kids/adults have a stiffer airway & the sound is rarely generated.Infectious croup is most common but there is a spasmodic croup that seems related to asthma and can come on without apparent illness.I would usually sent this kid to an ENT for an airway eval and 2nd opinion on diagnosis