Sink Your Teeth Right Through My Bones, Baby

I've noticed this too, but not just with fanfic. Also, p.o.v. changes. I think it's just lack of attention, and possibly also lack of editing/beta reading. A good beta should be able to spot this sort of thing, but I think a lot of people get excited about they've written and just want to get it out there, so skip that step. I am guilty of this myself :$