If you did not mark Confirm Logout in System Settings => Advanced => Session Manager then leaving your KDE session by K => Leave => Logout produces a problem. Instead of restarting your login manager, the whole system is shut down.

But as soon as active Confirm Logout, you reget a login surface when you logout.

This is a bug of KDE 4 and should be fixed.

Can you reproduce with a new user?

Yes, the problem is the same for a new user.

Please ensure your distribution KDE 4.3 packages are up to date, reconfirm the bug and report it at bugs.kde.org if it doesn't exist already.