Comment 43 for bug 36256

The creation of those files is done directly by startkde, not an external script.
Here is the bit of code that does it :
# configuration of the gtk_qt_engine if not already set

if [ ! -x /usr/bin/startxfce4 ] && [ ! -x /usr/bin/gnome-session ]
then
        if [ ! -e $HOME/.gtk_qt_engine_rc ] && [ -e /usr/share/kubuntu-default-settings/.gtk_qt_engine_rc ]
        then
                 cp -f /usr/share/kubuntu-default-settings/.gtk_qt_engine_rc $HOME
        fi

        if [ ! -e $HOME/.gtkrc-2.0 ] && [ -e /usr/share/kubuntu-default-settings/.gtkrc-2.0 ]
        then
                 cp -f /usr/share/kubuntu-default-settings/.gtkrc-2.0 $HOME
        fi
fi

You can have the issue if you installed kubuntu and then installed gnome of xfce.
There is not issue if you install kubuntu-desktop package on xubuntu or ubuntu