"Hermione slid out of her bunk and moved like a sleepwalker toward Ron, her eyes upon his pale face. She stopped right in front of him, her lips slightly parted, her eyes wide. Ron gave a weak, hopeful smile and half raised his arms.
Hermione launched herself forward and started punching every inch of him that she could reach" (ch. 19).
Hermione wondered if she was still asleep, still dreaming.
Ron was back.
He'd returned to them – apologized.
And the whole time she was screaming at him, accusing him, letting out all the pain and anger she'd kept holed up inside, she couldn't help but feel a little bitter that Harry was defending him.
It was just like old times – whenever she and Ron would argue, Harry was always on Ron's side.
And then she realized – it was just like old times. They had fallen into the old pattern, so easily she hadn't even noticed it.
Their triangle was complete again.
That had to be an incredible catharsis for Hermione - and I think she just needed an outlet. Ron's the person for whom she has the strongest feelings, whether affection or anger, but they all need each other to make the balance work.