VMkernel does not always shows the issue with reboot instances or why esxi host reboots unexpectedly.. If its user initiated or initiated from the powerbutton on ILO, you would see that in vmkernel.
For more info VMware Knowledge Base This KB gives you better understanding.
If ESXi host reboots itself every 20 mins, I have a gut feeling it can be related to hardware or temperature issue.
Please engage your hardware vendor and also check the SEL logs
Thanks,
MS
