Mae hit the man in the yellow suit in the head with the shotgun.
The man in the yellow suit is looking for the spring that made the Tucks immortal. He heard the story passed down through his family, and he believes it. When he hears the music box in the woods, he is convinced that it is real.
The man meets Winnie outside her house, and he sees her being carried away by the Tucks. She is therefore really worried about him finding her. She does not want her new friends to be found out.
"He knows me, though," said Winnie. She had forgotten, too, about the man in the yellow suit, and now, thinking of him, she felt a surge of relief. "He'll tell my father he saw me."
"He knows you?" said Mae, her frown deepening. "But you didn't call out to him, child. Why not?" (Ch. 11)
Mae Tuck is very nervous. She is not a violent person, but she has tried so hard not to let anyone find out about her secret. The man in the yellow suit does not seem well-intentioned. Mae is concerned about what he will do know that he has found the Tucks. He tells them that the Fosters gave him the woods, and he is to bring Winnie home.
Things take an ugly turn when the man in the yellow suit threatens Winnie. He tells the Tucks she will drink from the spring water and be his demonstration. This horrifies Mae, and she acts. She hits him in the head with a shotgun.
With a dull cracking sound, the stock of the shotgun smashed into the back of his skull. He dropped like a tree, his face surprised, his eyes wide open. And at that very moment, riding through the pine trees just in time to see it all, came the Treegap constable. (Ch. 19)
Unfortunately, she seems to have hit him hard. As far as the constable is concerned, it was murder. They will try Mae, and then she will be hanged. This is terrible enough, but Mae cannot die so hanging her will give away the family secret.