It does not actually do this. I suspect what you're seeing is the installer or, more likely, its X session crashing and the regular desktop session starting in its place. This is probably the result of bug #714829, which is fixed in the most recent versions of ubiquity, and present on the 20110301.1 and newer daily-live CDs.
** Changed in: ubiquity (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/723833 Title: Installer – If you pause for a short time on the 'do you want to install or try Ubuntu' install process screen you automatically boot into Ubuntu. -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs