I honestly feel that the stress from having her daughter undergo a surgical procedure, albeit a minor one, has her a bit on edge and dredging up some negative feelings about other things she's having trouble dealing with.

Another member in this thread brought up the idea that she may be harboring resentment toward complacency in the marriage and it not progressing to a place where she is happy with it, aka marriage. This might be a possibility, but there may be another reason that she's angry seemingly out of nowhere.

Ask her if she's okay and if there's anything you can do to make her feel better. Open and honest communication is key to managing a healthy relationship.