Trauma to the Blood Supply.
The blood supply to the amniotic vesicle along the lesser curvature is relatively fragile, especially for the early vesicle. Once disrupted the embryo / fetus loses its blood supply and dies. Rough handling can also lead to complete rupture of the amniotic vesicle. This is sometimes used as a method for terminating pregnancy in feedlot heifers.
Drost M (1982)