This is because, at some point, you clicked the Enable AutoLoad link in the Secure Webmail Login interface.

To disable auto-loading, perform the following steps:

1. Access cPanel's Webmail interface (Home >> Mail >> Webmail).
2. Click Go to Secure Webmail Login.
3. Click Stop before the timer counts down to 0 in the window.
4. Click Disable AutoLoad to make certain that the Webmail client does not automatically load whenever you access the Secure Webmail Login interface.