Its not something new. Mariah lip syncs parts of some of her songs after her prime, not the whole song, esp if she's got to move. Its not uncommon for artists to do that, even Celine Dion admitted that its being done before, its happening now and it will happen in the future, Mariah usually gets caught tho because she sucks at lip syncing. If you watched the whole clip, Mariah sang live after, even rapped ODB's part.
Compilation of singers lip syncing fail:
Compilation of singers lip syncing fail: