The code has, at https://github.com/kspalaiologos/bzip3/blob/bf2f0e02fd59f4c4... :

            } else if (argv[i][1] == 'j') {
                workers = atoi(argv[i + 1]);
                i++;

If the last argument is "-j6" then this will read past the end of the allocated argv strings and try to do atoi(NULL):

  % ./bzip3 -j3 < README.md
  Segmentation fault

"-j6" is standard getopt() behavior, and the default expected behavior from Unix/POSIX systems.