People have reasons that's why they will change their username and it could be privacy related or just for fun.
Whatever the reason, I guess if the username change is not announced publicly by the user or admin himself we should not be bringing it to the public.
Igebotz and The Sceptical Chymist have discussed it publicly but others are new to me.
Here is a take from theymos on discussing username changes
As I've stated previously in one of my posts, I don't care about the cases and reasons of others, but mine was for the sake of security, while a few others were for privacy reasons (I can't recall the exact reason), while Sceptical Chymist requested a new name, I assume he was bored with being The Pharmacist and desired a new identity.

I don't mind the public knowing that I used to be ighehh, and because my new name represents my brand, I'm fine if I go viral; if I wanted to remain anonymous, I wouldn't have asked the admin to include a custom title.