What falls under the category of negativity?

It's all subjective. If you feel they were negative then that's your call.

Its up to interpretation

As the others have said, it is subjective. For me, I tend to avoid using the report category for negativity, because it's so nebulous. But if I absolutely must use it, then it's when a person is just so absolutely pessimistic and unpleasant to play with, that it is ruining the entire mood of the match. Not just "we can't win," because that's an opinion, and they might be correct. But more along the lines of generalized insults towards a champion/role that are not verbal abuse directed towards a specific player.