Governor Christie is targeted because he's viewed as a front runner in the Republican Presidential Horserace and therefore fair game for attack. If the Democrats and their allies in the media can wound or destroy GOP front runners as they surge to the front of the pack, it will make the job of continuing to control the White House bully pulpit that much easier.
What disturbs me is the Republican piling on. So-called conservative's open disdain and harsh rhetoric used against Governor Christie is a tactic that may come back to haunt them when a candidate they favor, such as Texas Senator Ted Cruz, comes under the same intense scrutiny and criticism that Christie is undergoing, at which time they will demand that all good Republicans come to his defense. This seems to be the epitome of hypocrisy.