Doors are locked when in flight. The locks require power to remain locked, so any failure in this will unlock them, as will sudden loss of altitude and speed.

It would be impossible to open a plug type (inward opening door) at altitude due to the pressurisation as mentioned in other posts.

Cabin doors to manual/automatic is nothing to do this this - it's related to the emergency slides.

I comment as a former aeronautical engineer. Things may have changed in the 25-odd years since I left the profession.