As Neil observes below, roughly 10-15 percent of the Massachusetts electorate seems to have remembered that they're Democrats, love affair with Scott Brown notwithstanding.
Now, the only poll that matters is the one on election day, but you'd obviously rather be in Warren's current position than 10 or 20 points behind. I placed a bet that Brown would not just win reelection, but win in a walk, and that's money that (happily) appears like it might belong to the counterparty to this particular wager.
These circumstances may appear to be a bit of poetic justice for Chris Capuano and other high-profile Democrats who sat out the race, presumably because Scott Brown looked "invincible" based on extremely early polling, and hey it's Massachusetts so if I just wait my turn I get a free Senate seat, right? Had Capuano run, he would have clearly been able to leverage the state's Democratic machinery, raised enough money to produce a competitive campaign, and generally done the things you'd need to do to make a run at the race. Indeed, Massachusetts Democrats already have a template for winning come-from-behind races against personally popular Republicans. But only Warren was crazy enough to think that in politics, sometimes you have to engage in a challenging general election contest against someone you disagree with, instead of waiting for a primary that you hope you can squeak through and then coast in the general. And now she seems to be reaping the rewards of that particular bout of craziness.