Waning immunity. Pertussis & most other bacterial vaccines (meningitis, diptheria, tetanus, etc.) give the body a time limited period of benefit. By 6-10 years after the last vaccine, the decrease allows people to develop symptomatic disease & or pass it to others. Since pertussis is more a chronic cough with teens & adults, some may never know they have it.
And... Dr ferguson is quite correct. Also, there are several species of bacteria & viruses that cause a "pertussis-like" syndrome, but are not pertussis. These conditions are not as serious as actual pertussis, and are not vaccinated against.