I think a lot of reasons. One is to mark their territory. Another maybe is to cause some pain for the other person or just playfulness. It could also be a response to the pleasure/pain of intercourse for her.

Those question are something I think a study should be done on. I don't think there is enough modern sex studies being done.

it depends because some guys think its a turn on and some women do to lol

Sometimes it's just a reaction to the passion. They don't realize how hard they are scratching.If their nails are long and sharp they can sometimes claw the skin off your back, and it's very painful. But it's not deliberate.