I always wondered this question and the other day I sent the question off to DomainTools.com and this was the reply:

Question~ How is a record created for "whois history" for the Whois.sc/DomainTools.com whois system ?

"Technically, a record is created at your registrar each time there is a change.  So if you update anything on your record at all, it will create a new record.  However, what we notice is that the popularity of the site determines the number of records in our system.  Check out google.com.  Possibly the site in question sparked some new interest or came up on a search queries.  You may not have changed anything on your side; but, for some reason, people find it interesting and looked it up. No real magic, although you’d think."

One reason I asked this question is because I see some domain names with a lot more "records" for whois history then others but the "age" of the domain names can be pretty much the same. Secondly, a domain name that I own has a new record for nearly every day since May, then took a month off and then was back at it again etc. I had changed nothing at my registrar, so these records must be coming from the "However" part of the answer.

The "However, what we (DomainTools.com) notice is that the popularity of the site determines the number of records in our system" could also translate into when somebody searched your domain at Whois.sc or DomainTools.com as well.

Having a recent climb in this number for a domain name you own, might get you to do some checking and you may find out why people might be searching it. In my case, somebody has been posting on Craigslist and useing my domain for people to visit to see more info in regards to his job posting. lol Thanks for all the traffic.