Because BT have now set their email service to do that. There is no user setting that you can change to stop it doing that nor can you ask BT to allow it to stay logged on for longer.
Thanks for that. I thought that might be the case and it is very disappointing. Why does BT do this? I do not get this with my gmail accounts. There are just two of us here and the desktop is in my house on a wired link to the hub. The only way unauthorised access could be gained is to break in!
Or an open session on a web server because you are logged into it.