Fireplace burning in the middle of the night....weirddd?

So before bed my mom puts wood in the woodstove and pushes this thing back to cut off oxygen and makes it burn slowly... no flames just hot coals burning. So about 5hours later when im downstairs the fireplace is suddenly in flames like someone just put paper in it and it caught fire. Then a minute later the flame is out... but nobody put anything in the stove. What would cause the fire to ignite like that without anyone opening the stove door?