There are a few circumstances where a uterine inversion can occur and include:
Pulling on the cord with no contraction. If the caregiver tries to deliver the placenta while the uterus is relaxed and the placenta is still attached to the uterus, the uterus can be pulled down and inverted.
Pushing on the top of the uterus when the uterus is relaxed. Forceful pushing on the woman's belly and top of the uterus when no contraction is present can cause the uterus to invert. (Pushing on the top of the uterus is different from gently 'rubbing' the top of the uterus to stimulate a contraction).
Partial separation of the placenta. Sometimes only part of the placenta separates and as the caregiver tries to deliver the placenta, because the bleeding has started to indicate the placenta is separating, the uterus will be pulled down with the part of the placenta that is still attached.
Pulling the attached placenta down during a 'manual removal of placenta' (MROP). If the placenta is reluctant to separate and is retained inside the uterus, the caregiver may try to remove it manually (with their hand). Pulling the placenta down that has not readily separated, can occasionally bring the top of the uterus down with it. You may wish to read more in
Retained Placenta.
An unusually short cord. If the baby's umbilical cord is unusually short (20 cms or less), the top of the uterus can be pulled down with the complete birth of the baby.