Supporting documents for British passport Renewal

[Guscoint]

Hi guys
I am a bit confused, I went to renew my passport and I have been told we need further documents !!
This is not my first UK passport and I was wondering why they are asking for more documents ?

Background
I was born in Lebanon my father was born in Sierraleone, I acquired BOC than registered as BC, lived in UK for 16 years and married.

The letter I received stated

I m writing to confirm that your passport application is under review by a passport officer who is carrying out essential identity checks, we are committed to completing these as quickly as possible.

In order that your application can continue I will need you to forward documentary evidence to establish your claim to British nationality , therefore you will need to forward the following

Original or certified copy of your birth certificate
Your father’s birth certificate
Your parents marriage certificate
Your paternal grandfather’s birth certificate
Your paternal grandfather’s marriage certificate
All passports, whatever nationality

Any help would be great
Thanks