The Bears secondary is injured and not very good, and so this was bound to happen eventually: Nathan Vasher was beaten badly by a far superior (relative to his position, anyway) Greg Jennings, and the Packers retook the lead for the first time in several quarters.
In the meantime, Jay Cutler had a chance to engineer a last-minute touchdown drive. Instead, he threw his fourth pick of the night, and the Packers were able to end the game with a kneel. 21-15 is your final score, and yeah, you know? That was Week One football. (If I was an even bigger d-bag, I would have written "weak one," which I'm now really glad I didn't do.)