Imus always disliked the Clintons and spoke accordingly. That is why Media Matters got on his case.
It’s worth noting that, in an aside about a week earlier, Imus had responded to a question about having Hillary on the show by indicating that he never would because ‘she’s the devil incarnate’. It produced enough of a ripple in the Clinton camp that Hillary made a public statement that she’d never go on the show. I suspect when we finally get to the bottom of this, we’ll find you-know-who’s hand on the controls.