There's cables inside the box and you might have bumped it enough to make the cable touch the fan blades and then it went back into place after the book propping and removing the book.

often the extra length of the cables just hang loose inside and can flop around if the box is bumped or tilted.

here's a picture -but it has the cables folded and tucked in nicely out of the way-
http://www.building-your-own-compute...ase_inside.jpg