Lots of reasons, some good, some bad. I'd argue the main reason is the dreadful treatment of rape we saw in Season 4, people have lost any kind of faith in the show's ability to deal with what is a difficult and sensitive subject with any kind of respect or maturity. It's worth noting that at this point in the show, three of the four main female characters (Cercei, Dany, Sansa, Arya) have been raped where as AFAIK none of them have in the books so far. When D&D keep restructuring the plot seemingly for no reason other than to include a rape scene then you have to ask yourself questions.
I'd also argue that the Sansa scene was like 100 times better than the Cercei/Jaime scene or the Casters Keep one, but the "rape, lol" attitude of previous seasons made people assume the worst. For which I don't really blame them.
There's also the fact that it did it's job too well. The show expertly ramped up the tension, the idea that something horrible was going to happen and you're just waiting for the other shoe to drop, since Sansa set foot in Winterfell then managed to make the entire sequence, when it came, thoroughly horrific. I think some people just found it a little close to home. For which, again, I don't really blame them.
So now they've put themselves in a situation where a scene which should rank alongside the Red Wedding and Oberyn's death has one of the most shocking scenes in the series instead they get a reaction more along the likes "... they're going to do another rape scene, aren't they? Yup. Yup. There it is. Fine. I suppose that would happen in these circumstances. Fine. *sigh*