Final answer:
The most likely cause of the sudden 'gush' of bleeding from the vagina while walking after giving birth is uterine atony.
Step-by-step explanation:
The most likely cause of the sudden 'gush' of bleeding from the vagina while walking after giving birth is uterine atony. Uterine atony is a condition where the uterus fails to contract effectively after childbirth, leading to excessive bleeding. It is a common cause of postpartum hemorrhage.