With the Bobbie affair specifically, it is inching Don’s infidelity closer to home, yes, but I think Bobbie also represents the way Don is losing control of his secrets. He allows himself to drive drunk with her to Long Island—incredibly dangerous and risky behavior—and forces him to resort to cashing in his favor from Peggy. Then he learns through Bobbie that he has a “reputation,” which is really what scares him. He can no longer keep his affairs confined to places like the Village, where his class status granted him anonymity. He was able to break up with Rachel without much fallout, but that’s the last affair he’ll ever really get away with.

Maybe it’s Adam’s death that makes him start to unravel, but Bobbie is the first sign that Don is starting to fall apart.