I had to do some research to remind myself why Peggy was so hostile towards Don. Had totally forgotten a lot of significant stuff.
Maybe I shouldn't fast-forward through the "Previously on Mad Men" part at the start. I mean I don't know if they even tried to cover it but in case anyone is in the boat as me, here is an attempted explanation:
Their last significant interaction was when Peggy and Ted had cooked up the Saint Joseph's Aspirin ad that was a tribute to Rosemary's Baby. Peggy was convinced it was going to win her a Clio Award and Ted, who was still all googly-eyed over Peggy, was letting her do whatever she wanted. They were going way over budget, hiring tons of actors, and so on, all without getting the okay from Saint Joseph's first.
Don saw what was going on and was annoyed with both the romance between Ted and Peggy and their shared un-professionalism. So he humiliated them during a meeting with Saint Joseph's, telling Saint Joseph's that the reason Ted was so passionate about the Rosemary's Baby idea was because it was rather...personal to him. Don kept goading Ted to explain his reasons, and both Ted and Peggy looked like they wanted to disappear. Then Don went on, "This was Frank Gleason's last idea", which was a complete lie. But it effectively took away Peggy's power as the brains behind the ad, embarrassed her and Ted, AND damaged the ad because Saint Joseph's didn't agree to the budget revision anyway, if apparently all the passion was for a dead guy and not the idea itself. Peggy's response to Don, later that day: "You're a monster."