We had a strange issue that any time a VM was snapshotted it then forced a reboot and sysprep. Reviewing the customization logs(C:\windows\temp\vmware-imc\guestcust.log) I noticed the following error..Unable to set customization status in vmx.
Upon further research it seems this is a known issue and is resolved in ESXi 5.1 Express Patch 5 KB2077640 as referenced in this article KB2078352.
However the hotfix only fixes future occurences, you still need to fix all the VMs affected.