That's the last resort comparison. When comparing two lines, if all the keys compare equal, then as a last resort, a basic string comparison of the whole lines is performed (
-r still applies but not the other options). That behavior is specified by POSIX:
Except when the -u option is specified, lines that otherwise compare equal
shall be ordered as if none of the options -d, -f, -i, -n, or -k were present
(but with -r still in effect, if it was specified) and with all bytes in the
lines significant to the comparison. The order in which lines that still
compare equal are written is unspecified.
sort, that last-resort comparison can be disabled with the
¹ as the sorting algorithm used by GNU
sort is otherwise stable, so disabling that last-resort comparison results in a stable sort..