Why does Maggie go to the attic? What happens when she sees Tom in the attic?

Maggie flees to the attic in tears because Tom has gotten very angry with her. His anger is ignited because she let his rabbits die while he was away at boarding school. Though she cries and begs him to forgive her, he says he will never forgive or forget.
The attic is Maggie's retreat, so she runs there to be alone and cry. She loves Tom dearly, and his anger hurts her terribly. This is not the homecoming she expected.
Tom goes up to the attic when his father orders him to find Maggie and bring her to tea with the rest of the family. When he arrives, Maggie flings herself on him in tears and begs Tom to forgive her and love her. He's moved on from his anger, so they both hug each other and nuzzle in an unabashed way, as children do. Eliot compares it to horses nuzzling. Then Tom, who still has his plum cake in his hand, says,

Don’t cry, then, Magsie; here, eat a bit o’ cake.