The paradox in this episode comes from the fact that time travelers (Rose and the Doctor) changed something they were only supposed to observe... not change. The Doctor makes reference several times to fixed points (although, he seems to pick and choose) and how it is part of being a Time Lord to be able to distinguish. That being said, he specifically tells Rose that she can see her father, and be there when he dies, but can't keep him from dying. When she saves him, it is changing a fixed point and not only creates a paradox but destroys the timeline. The paradox is the focal point, so that's why the reapers try to fix it. However, if you notice, they don't just go after the paradox. They go after anything and everything in that timeline. I think from a writing perspective (although never said) it is to keep from that timeline converging with another timeline (say the alternate reality where he is still alive and Rose is a dog) thus destroying both realities. There is a theory in science about two universes colliding, one that follows string theory, that states we are eventually going to run into another universe just like our own and it will destroy us both. Probably, the reapers were just there to ensure the survival of at least one universe.
Now, with that being said, it could have also been the writer trying to build up their characters for the series early on. Most writers for TV shows will use one-off story lines to build on a character they want the audience to like. Rose and the Christopher Doctor were trying to make a footprint to get the show going again, so we had to fall in love with them. Otherwise, the show wouldn't have lasted and we would have missed out on Ten (which would have been a shame).
So, a mix of one-off writing and a fixed point in time being changed brought the reapers in, and we will likely never see them again.