Raindrops are actually falling down vertically and if a wind current is there it will be slanted. The more the wind, the more diagonal it seems. But here the question is why it is slanted or falling at an angle even without a minute breeze?! It is nothing but the rotation of earth causes such an illusion to us that the rainfall is slanted or falling sideways. Even though the raindrops are also in the earth's magnetic field, there will be a small distortion when it is falling which make us feel like rainfall is not coming down straight. Yes a small inclination is there even while it's raining pleasant without any air current.