Your assertion that Gravis is the source - and the
only source - of the Hillabounce is clearly right on second glance, but it seems provably wrong to say Gravis has a house effect for Hillary. You say that +18 is anomalous and because it's in between a +9 and a +11, that is literally correct. But ignores that behind +11 is +
32, +14, +22, +18, and +18, all within December. +32 is clearly a house effect - it's some no-name college which 538 doesn't even bother to rank (and which is probably not subject to 538's controlling for house effect for that reason). Let's also ignore Fox News' +14 just because, even though they're a pretty solid B at the 538 poll ranking. The remaining polls are Monmouth University and PPP, which are both reasonably (and in Monmouth's case, highly) regarded.
http://fivethirtyeight.com/interactives/pollster-ratings/
Gravis may well be full of shit - 538 ranks them at a C - but it's clearly untrue that they're
consistently full of shit, or at least that they're consistently shit in favour of Hillary (ctrl-f Gravis
here to verify this). They've been reporting results well within the norm throughout the race. Something anomalous is happening here. That something could be an unbelievably, anomalously shite poll - or something else.
2) Also, it's not quite only Gravis. PPP's latest poll - the one that 538 is ranking highest in their methodology - has Hillary up 6, just over the +5 lead she has in the poll of polls. This is a weakly positive result which 538 is counting as a "plus" for her win probability. (Am I doing the maths right here?) At the very least, it's an arrest of Sanders' momentum.
3) I strongly suspect that Gravis, like PPP, is picking up
some signal - but mostly noise. These kinds of huge swings are exactly what volatility (actual voter opinion volatility, not pollster noise) looks like. It's a bit weird for political junkies (like ourselves), but as 538 and you correctly point out, most people simply don't give a shit about politics until just about now, and will change minds rapidly and frequently till then.
4) Thanks for digging your source up - sorry to make you do it.
Ed: alternative theory - Hillary down recently is due to Trump's recent attacks on her and her husband. He's a shitmonger, but as my roommate's Internet history convincingly demonstrates, there are plenty of people out there with an appetite for swallowing complete shit. Attacking like a kindergardener worked on Bush, Carson and seemingly Cruz so far - it's not obvious to me that Hillary is completely insulated from this.