Have you checked the hostd log on the host to see if there is any indication why it restarted? Maybe check the iLO/DRAC also?
You can search hostd for "reboot", "shutdown" & "poweroff"
When the host is online, you can try to stop hostd and vpxa service and see if this stabilizes the ESXi host. If it does, it means that something is causing hostd to crash which in turn is affecting the host.