Background Cronjobs are turning into Zombie processes

Question

I have 60 cronjobs scheduled to be run after reboot as background jobs as mentioned below:

@reboot sleep 900 && /home/creta/Scripts/N1/./N1_Docs.sh > /dev/null 2> /home/creta/Scripts/N1/N1_Docs.log &

All these background scripts are turning into zombie although they are working as expected.

Scripts as shown below:

#!/bin/bash -xv
while true; do
    for z in "/D1/N1/*.pdf"; do
        /usr/bin/rclone move /D1/N1/ New:D1/N1/ --log-file=/home/user/Scripts/N1/N1_Docs.log
    done  
    sleep 18
done

Why are they turning into zombie and how can I fix it?