Why does my Macbook Pro try to shut down when I leave it unattended? When I close the lid of my Macbook Pro (Lion), it goes to sleep, which is good: it powers down without closing my applications.

But if I just leave the computer unattended for a while without closing the lid, it attempts to shut down instead. It usually fails at this, because there's often a program such as Photoshop (which doesn't use Lion's autosave feature) blocking shutdown with a "Save as" dialog. But it does always succeed, annoyingly, in closing my browser etc. It does this even when the computer is plugged in and fully charged.

When I leave my computer unattended for 15 minutes, I want it to behave exactly as if I had closed the lid: it should go to sleep, not shut down. 

How can I do this?
Try going to System Preferences and choosing Security &amp; Privacy.  When that application window opens, choose "General" tab and then look to the lower right corner and click on "Advanced".  Uncheck "Log out after _ _ minutes of inactivity.

Not sure but this might be the cause.

Good luck