There are many avenues a Registrar can take to separate a Registrant from his valuable dot com domain.
One is, to modify the whosis, so that it is out of compliance.
Two, withhold Renewal Remainder notices the Registrant is used to receiving. If the Registrant depends on the renewal notice to be reminded, he will let the domain drop.
Three, change the credit card info, so that auto renewals don’t go forward.
Four, put a bogus expiration date in the Renewal notice.
The latter two happened in the case of iSound.com.
Five, ICANN decredits a Registrar, and the domains are sent to an Insider Registrar, or split among receiving Insider Registrars. Some domains get lost in the shuffle, especially if they were part of a Reseller account. This has happened with RegisterFly.
Six, a Registrar that offers hosting syncs payment of hosting a website to renewal of the domain name. If a Registrant chooses to switch hosting, he may lose his domain name.
Seven, trumped up violation of domain use.
Eight, Registration Terms of Agreements are now unfairly burdensome to the Registrant, forcing him to agree to pay for lawsuits involving the Registrant or his domain, whether they are resolved in the Registrant’s or Registrar’s favor or not, while Registrars maintain the right to point a domain to their own content, while the whois info still shows the Registrant’s contact info. They state the Registrant has to put up a bond.