I think the lab/person that developed the negative scratched them. This can happen if you run your fingers down either side of the negative to remove the chemicals. I'm not sure how a machine develops the film, but after this happened to my negatives a bunch of times, I stopped taking it to that shop and lo and behold, the problem never happened again. The icing on the cake was when the owner of the shop said that it was my camera that needed a clean, obviously not realising that I have a few 35mm cameras.