Apparently, "buckraking" refers to a journalist taking a lot of money for a speaking engagement, especially speaking to a group that has a particular agenda; such a payment may cast doubt on the journalist's objectivity.

Despite the etymology (a portmanteau of "buck" and "muckraking"), the term doesn't actually have anything to do with muckraking; the word "muckraking" is apparently a stand-in here for "journalism," plus of course the pun/joke that the journalist is raking in the bucks.

I'd never heard the term before, but apparently it goes back to at least 2002, when NBC banned it. (Though the phrasing of that piece is ambiguous; perhaps NBC banned the practice under a different name.)