The site will automatically log you out if you have not used it for 2 hours. The site detects usage each time you click on a link or a button. Having a page open on your device does not in itself allow the site to detect usage. There needs to be interraction between your device and the site's server.
The 2 hour time-out provides what we believe to be an appropriate level of security. These days, the web isn't just available on desktop machines, it can be accessed on a multitude of devices, many of them mobile. These items can be lost or mislaid, so it's essential that password protected sites, such as ours, implement automatic log-outs.