If you're in the US like I am, I think it's because the BBC redirects all non-UK visitors to BBC.com, and so when you hit the back button from BBC.com you get sent back to BBC.co.uk which in turn redirects you to BBC.com. It gets on my nerves too.
I first saw this on Facebook but I have since seen even sites that I used to respect follow this same pattern.
To me this is nothing but another dark pattern.