Obviously some of it comes from the odd comment by the likes of James Carville. But what about the blind quotes in stories, like today's WaPo front-pager, about all the indignities Hillary has suffered?
My theory: A lot of them come from bitter Clinton moneymen. These are people who joined up early so a future Clinton White House would be sufficiently grateful (and reciprocate with ambassadorships and seats on federal boards and commissions). They made significant investments of time and money. And once their investments fizzled, they were basically left with nothing. A losing operative or policy person can go work for the nominee--in fact, the primaries may have raised his/her profile. But while a money person can always cut another check, they've often missed their shot at real influence.
Worse, the Obama campaign is an especially harsh place for refugee donors. Unlike other campaigns (including Hillary's and Bush's), the Obamanauts don't hand out a lot of highfalutin titles, and they don't keep track of bundling efforts in quite the same obsessive way. So the Clinton ex-donors aren't even getting the head-pats they're used to. (That's not to say the Obama campaign isn't appreciative--just not nearly as into gold stars.) My sense is that, after Obama clinched the nomination, a number of key Clinton moneymen played coy in hopes of increasing their leverage. But, as usual, the Obama people didn't play ball, and most of the rank-and-file donors and fundraisers came aboard anyway. It's the kind of thing that might rankle if you were a Hillaryland bigshot.
On top of everything else, the moneymen tend to fall outside the usual campaign hierarchy. They don't feel as constrained by campaign omerta and are chatty with reporters even when things are going well. I'd guess they're especially talkative when they feel aggrieved.