What's wrong with Dolly Parton's voice?

The Dolly Parton feature, for the most part, was okay... until she sang her solo lines. Is it just me, or was her voice sort of wobbling throughout her solo lines? She sounds fine in the rest of the song, but when she gets to, "And them bell-bottoms I wear make walking out hard to do," (and the 2 lines that follow) her voice does the wobble thing. Is there a reason for this that I'm just oblivious to?