Did Diana need a reason to say unkind things? Not really - she would lash out at anyone who she felt like e.g. Tiggy Legge-Bourke and the abortion comment.
Exactly, it's been known that one day, someone could be Diana's dear friend and the next morning he/she was totally frozen out for an unknown reason. She told Sophie at one point that Sophie would never make it in the Royal Family and she was just being lead on. Remember also, this was in the Ninetie's and the War of the Wales was at the height of it's ugliness. Sophie was beginning to receive quite a bit of press coverage as Edward's girlfriend because it was beginning to be assumed that Edward would never marry. I won't get into it because that's off topic for this thread.
I believe that Diana was feeling as if she was taking second place in behind Sophie in the press and the Firm as well, when Tiggy was taking care of her sons when they were with Charles she felt that her place as a mother was jeopardized. Neither was true, but then she lashed out at Sophie and Tiggy with the cutting remarks.
Was Diana right in her actions and did I support her? No, it was irrational and immature and when these revelations among others came out, I think it did tend to tarnish her some.
Sophie is a favorite of the Queen as she regularly gone riding with Her Majesty and has shown support to the Queen and established a genuine relationship. The Queen tried to support Diana and was fond of Diana, but had grown to becoming bewildered at her actions and then after the Panorama interview, Diana lost the Queen's sympathy.