I personally have found that when ever you're around a group of people with a different accent you tend to pick it up even with out noticing it, and then when you go home, you have it for a while until your friends basically tease it out of you.
I had a southern accent as a kid, very strong, mainly picked up from my father. I purposefully worked on my accent to get rid of it, based on my mother's midwest accent and TV shows I watched lol. As I got into higher levels of school in Florida and around a larger group of people it was easy to keep it away.
Working in customer service in my home town in North Florida, people would ask me all the time where I was from because I didnt have the right accent (guesses being Canada, Pittsburg, Denver (close, mother is from there). Moving to Nashville I find myself slipping into it just from being around it. Cant seem to help it lol
In my early 20s I spent a lot of time with my best friend, who is a gay male, going out to clubs and stuff. My boyfriend at the time would get so annoyed with me because he said I picked up some of the lispy tones lol.
I think it just happens to ya naturally if you expose yourself to different regions and types of folk. Would be a cool project for a science or social studies fair lol
On topic, I watched this episode of Graham Norton too lol . She looks like she needs a sandwich and her hair colored back dark. Would never have known it was her had I not read it in the guide info