You didn't specify in which chapter this takes place, but I assume it is at the end of Chapter 3. Max has just helped Kevin retrieve his ornipthopter from the tree, and he invites Kevin back to visit the down under--Max's basement bedroom. Max wheels Kevin in his red American Flyer wagon, and they head downstairs. Kevin tells Max about his mother, the Fair Gwen, and talks about King Arthur and his magical sword, Excalibur. When they hear Gwen calling for Kevin, they head upstairs. But when she sees Max, Gwen grabs Kevin, carries him up the stairs, places him in the wagon, and "almost runs home." Kevin's mother does this because she is shocked at Max's size and is always worried about Kevin. Max decides that
It's pretty simple, really. She's scared of me.
Gwen apologizes later, and Max finds out that she was good friends with Max's late mother.