It's obvious Goodyear brought a harder tar, but I wonder why no one has really mentioned the smaller fuel cell. Less weight, rear end breaks loose, and she'll come around everytime.
I don't remember if they measure the weight with a full or empty tank (I should've asked one of the inspectors the last 2 years). If it's a full tank, it's the teams' placement of the additional weights to bring the car up to 3400 pounds (plus the penalty for a light driver) that would make the car break loose. If it's empty, then the cars are about 33 pounds lighter (give or take a pound) over the rear axle.
In any case, there is, if I recall, some on-track adjustability of the weights. I don't know how much time the Busch teams had to adjust before impound though.
One more thing to consider; it's a LOT warmer in Vegas now than either the average or when they tested. That heat makes for a greasy track.