Check the vmkernel.log& vmksummary.log files in the /var/log directory (via Shell or SSH access). If there is any related event about host safety reboot, it's happed normally.
But if you find the startup events suddenly after normal actions/operations, so there must be a crash event that is happened in your host. You can also configure coredump to collect dump information about PSOD events on your host. Check the following links to find out how to configure it:
Undercity of Virtualization: What is the VMKernel Core Dump - Part I
Undercity of Virtualization: VMKernel Core Dump - Part II: How to add & remove coredump file