Why Process/program becomes zombie?

If script is running fine from command line then, why the same script becomes zombie state after running through cron and How you will troubleshoot the same ? Here following real example : [root@abc ~]# ps ax | grep Z 23880 ? Zs 0:00 [checkloadadv.sh] 23926 pts/0 S+ 0:00 grep Z [root@abc ~]# strace -p 23880 attach: ptrace(PTRACE_ATTACH, ...): Operation not permitted [root@abc ~]# pstree | grep checkload init-+-crond---crond-+-checkloadadv.sh [root@abc ~]# bash /usr/bin/checkloadadv.sh System Load is OK : 0.05