Reboot Required next to host name?

We are running vSphere 6.5.

I logged into vCenter today and went to the cluster level. 

Typically I am at the VMs and Templates level.

Next to an ESXi Server in the cluster, i see a the following

  ESXi (reboot required)

I spoke to some Admins and they stated no patches were applied.

Any idea what may have been done on the ESXi host that would cause a request for reboot?

Are there any files to check?

thanks