Finally, I found the reboot is actually expected in my case. In this case, guest VM customization is used. The sequence seems like: pre-customization script -> reboot -> post-customization script. Cloud-init is also used to apply some other configurations and that takes a little bit longer. The cloud-init may or may not get interrupted by the reboot, this is why sometimes I was aware of the reboot, sometimes not. VMware folks suggest to use either traditional guest customization or cloud-init, not both of them at the same boot.
