It has been shown that salpingo oopherectomy as well as child birth can rarely excite autoimmune hypophysitis. what is the likely reason these hormone shifts can lead to autoimmune hypophysitis?

Auto-antibodies: Like any auto-immune diseases eg lupus etc) we don't know the exact reason as to why it happens and how it is triggered--- and hence there is no cure.

Rebound phenomenon: I am not aware of oopherectomy causing autoimmune hypophysitis, but after pregnancy many autoimmune disorders "flare". Pregnancy is a state of autoimmune suppression (this makes sense otherwise you might reject the baby), but once the hormone levels drop, it somehow triggers a more heightened immune responsiveness.