It is worth noting that the passport interviews were called for
all first adult British passport applications, including for those born British citizens and those who had already held British passports as children/minors.
Because even those born British citizens are called for these interviews, you will find this as a fairly common topic on forums oriented towards students, when one is likely to be applying for their first
adult British passport, such as
TheStudentRoom. Searching "TheStudentRoom passport interview" and similar terms in a search engine will give you a broader idea about these interviews.
When I naturalised, it was a compulsory interview for all first adult British passport applicants and I was lucky that I was able to arrange an interview with a passport office just down the road from my workplace.
Nowadays, it is a random selection from such applicants. But again, worth remembering that all first adult British passport applicants, including those born British citizens, can be called for such interviews.