Inside that raindrop, the coloured rays collide with the inside wall. This wall acts like a mirror, so it makes the rays of light bounce off it. So they bend still more, and shoot back out of the raindrop through the same side they came in.
The sunlight had originally come from behind you, and now, the changed light is coming back towards you. Your eyes see an arc of beautiful colours reaching across the sky, reflected by thousands and thousands of tiny raindrops!