In August 2005, L'Equipe announced they tested the B sample from one of his 1999 tests, which had been frozen, and it came out positive for EPO (those tests were not available in 1999). The A sample had not been frozen. The ICU did not think it was enough to impose any sanctions, but that finding got the ball rolling.
I am still surprised he has decided to stop defending himself, which amounts to a de facto admission of guilt. However, it's not clear yet he will be stripped of his titles. The USADA has no authority to do so. Only the ICU can do it, but certainly now it will be very hard for them not to do it.