The Doctor as we know can travel any-were in time and space but if the energy field is strong the Doctor cant control the T.A.R.D.I.S so the field draw him.So that is why the Doctor is here.T.A.R.D.I.S's power felt and the engines are started to don't perform right so it goes to emergency landing and it drawed by the magic field which is stronger and the field is the cause of the Doctors appearance at the forest of Hogwarts school.Magic can make people get crazy but for the doctor is something new and the cause of that is that the person for example as the Doctor is to have trouble because the rules are unknown for him so he can live or fit perfectly in that new area of living conditions.