Reading the preview, her behavior doesn't seem that unreasonable, and especially not for Wonder Woman on a bad day.
Sure, Diana may have smacked Giganta around partly because she was pissed off. But Giganta, a known super-criminal, was apparently stomping around while giant sized and in her supervillain suit. Wonder Woman did tell her to knock it off before attacking, and she did stop fighting when Giganta made it clear she wanted to talk.