Another Abbey Road type photo happened two years ago on this date (September 19, 2015), this time with Doctor Who! I just know it was Peter’s idea since he’s a huge fan of rock n’ roll and The Beatles.
At the 00:29 mark you can hear Jenna yell to Peter, “Right foot!” when he had his left leg forward (the same as John Lennon). My guess of the photographer wanting to switch is to match Jenna’s footing (in place of Paul McCartney who had his right leg forward). I think the switch in Peter’s legs was made because the photo is intended to capture Clara following in the same footsteps as the Doctor. It’s anyone’s guess, I just think how Series 9 played out with her being more reckless, she was a lot like the Doctor, and it eventually led to her untimely death.