alt Hacker NewsMy guess is that they ran selective search on the domains which get registered with any registrar, thats the trigger to start the search. .gov domains are not managed by your typical registrar which is selling the domain registration information to all these downstream partners/scavengers (for lack of better word)
The OP says it's using CT logs, not new domain registrations. The approach you have in mind would not include subdomains and would be less likely to coincide with a new server being configured.