Microsoft has said the design flaw causing Xbox Live accounts to be wiped beyond recovery, and losing all earned Achievements, is being investigated. The company issued a statement after a number of people moaned about the problem, which can be diagnosed by the '8015d080' error code when trying to resubscribe. The problem comes from Windows Live ID email addresses, such as Hotmail accounts, expiring and being deleted. These then disappear along with the associated Gamertag. A Microsoft customer service rep told GamesIndustry.biz there was no other way around the problem than to set up a new Gamertag with "your preferred email address". Unfortunately Achievement points are non-transferable. Whoops. Source
Sounds like it will only happen if you haven’t logged in to the passport account you used to create your gamertag for a long time. I think your account gets deactivated after 9 months, could have changed now.