What triggers the Users.Update Date field to be updated?

Summary

Not every update will update this field.

Content

Had 204 users that were Removed as of 10/15/2018 who became Approved last week, but their Users.Update Date field shows 10/25/2017, the same as it was on 10/15/2018.

It is disturbing to me that 200+ people can have their accounts reactivated but I cannot find out when they were reactivated.

Am I looking in the wrong place?