Given Susan's single-mindedness when it comes to Rat and Crow business, I have been sort of assuming all along that the problem is with both of them: Susan pushing Peter to get involved in things that make him horribly uncomfortable, as well as sort of shoving her involvement in his face, trying to force him to acknowledge the necessity of it and to hear him say he accepts it. Peter, being both stubborn and the High King, doesn't exactly respond well to this, and so ends up being more critical of her than maybe he would otherwise.

Which would also explain the difference in Peter's attitude between Susan and Edmund - Edmund respects Peter's boundaries (limitations?), and doesn't flaunt the Rat and Crow business in his face, nor does he try to manipulate Peter into doing what he, Edmund, thinks is best (or if he does, he is better at it). Whereas Susan can't keep herself from thinking she knows best how everyone's life ought to be run, and if they don't get there on their own, well, it's up to her to shove them in the right direction. Result: tension!