There are two reasons that seem to be escaping commentary so far.
First, Krugman and his fellow travelers have been waiting to tell this story ever since the summer of 2009, when the argument over health care reform began in earnest, and even earlier. They had a sort of Political Mad-Lib in hand. They knew the narrative they wanted to tell—that conservative politicians and commentators were inciting violence against liberals—and were just waiting to fill in the names and places.
Krugman himself admits this. He has been "expecting something like this atrocity to happen . . . ever since the final stages of the 2008 campaign." He cites "the crowds at McCain-Palin rallies," which the Left accused of being particularly vicious and violent. Then, in the midst of the extended health care debate from the summer of 2009 into early 2010, liberals were upset about the anti-reform protests and warned that a rash of violence against the supporters of Obamacare was imminent. When the bill passed, a number of Democratic lawmakers, as well as two Republicans, faced threats and vandalism. Elected Democrats spoke of domestic "terrorism." Surely comments such as those of Michelle Bachman, when she said her supporters should be "armed and dangerous" on the issue of the energy tax, were "fanning the flames with coded rhetoric"? Tea Partiers were called racist, homophobic, and xenophobic; they were blamed for Obama-Hitler posters carried by far-left Lyndon LaRouche supporters, and for signs carried by "Tea Party crashers" seeking to discredit the movement.
As the Tea Party movement remained strong through the 2010 elections, the warnings continued, but a problem arose: the predicted wave of violence never materialized. The Tea Partiers turned out to be precisely what their defenders contended they were: reasonable people who sought change through peaceful and democratic means. So Krugman et. al. were waiting for cases of violence, lest they be discredited. They saw the horrific slaughter in Arizona—and leapt at the chance to vindicate themselves.
The second reason for this eagerness to blame the Tea Party Right is because many liberal commentators have believed from the beginning that they are irrational bigots inclined to anarchist violence.
Conservatives are right to respond that Democrats have used the same cross-hairs and bulls-eye imagery (Hot Air), that they have placed conservatives in the cross-hairs in campaign commercials (Intellectual Conservative), that they have burned Sarah Palin effigies and threatened Bush repeatedly with assassination, and that Obama himself spoke of the foolishness of "bringing a knife to a gun fight" (WSJ). Journalists themselves use military terms frequently to describe political battle (Howard Kurtz), and it's laughable for Krugman and Markos Moulitsas and Ezra Klein to complain about incendiary rhetoric when they indulge in so much of it themselves. The Left created its own climate of hate over the past ten years (Michelle Malkin assembles the evidence), and "former Gov. Palin and the tea party movement are more the targets than the source" of political hatred (Glenn Reynolds).