I seem to remember saying a long time ago that the way we're set up, with the emphasis very much on attack, we may not lose many, but when we do, there's a chance we'll lose badly.
Apparently, Peterborough play what amounts to an old fashioned 4:2:4 and their 4 were up against our back 3, sans Dan Scarr. I haven't yet quite understood Saxon Earley but since he's played three, scored 2, it looks odds on he likes to get forward. In days of yore, Bali Mumba would have been given the No 7 shirt and called a winger (remember those?). We responded to going 2-0 down by taking off a more defensively minded midfielder in Butcher and bringing on Azaz. We took off Earley, leaving us with a back 4 including Mumba.
Stephen Schumacher has talked about individual errors. The penalty that started the rot was apparently caused by a failure to pick up a man from a free kick which Earley tried to redeem by a blatant push. He also referred to collective failings. It's also though the way we're set up and our mindset.