The question says the ball is "covered in carbon coating." So when she "pulls the ball so that it touches one of the plates," the ball absorbs the charge that the plate was. Due to electric fields it will accelerate to opposite plate ( so on...), and it will then touch the other plate, same thing happens as before.
This is what my teacher said anyway. Surprisingly, the above answer isn't even in the markscheme, but that explains the graph imo.