What would cause a child to grow several inches within a few months?

My niece is ten. Over the past year, she's grown almost eight inches! I see her about once a month, and every time she's noticeably taller. She's 5'-6" now. My brother and his wife are on the low end of tall. She can eat us all under the table, but she's remained as skinny as a rail. She hasn't yet begun to develop in any other ways that would signal the onset of puberty, but her voice is fairly low. Is that related to the height in some way?