It is British, from Cricket (as British as it gets):
on the back foot
(cricket, of a batsman) Having the weight on the back foot in order to play a defensive stroke
(idiomatic) in a defensive posture; on the defensive; off balance; at a disadvantage. quotations
https://en.wiktionary.org/wiki/on_the_back_foot
“‘She wanted me to be on the back foot and she succeeded. It felt very deliberate as opposed to an oversight.’”
It now makes perfect sense - thanks Doc!
Thank you for the insight on the term. Makes it more clear.