Uh, The only way your achievements would get deleted is if you modded your gamerscore. This I know for a fact. No way that can get glitched.. Your rank can be glitched by getting a corrupt player file, which also, is ruined by modding. OR you were simply just unlucky. But seeing how your gamerscore was reset, it looks as if you were modding.
That is my first theory..
But if you are meaning it in a different way... Your Downloadable content can only be used by the account it is downloaded on.. Dumb I know, but that's how they made it.
That's theory 2.