What causes session timeouts?

I have users complaining that their sessions are timing out from inactivity, even when they are working in them. In my projects I have the setting for session inactivity timeout threshold set to 15 minutes, but I am getting the complaint from some of my users that they open up a new tab and see the session timeout within minutes of opening that new tab. I am wondering what exactly causes sessions to timeout and how can I mitigate this issue with my users?