One would hope that fifty or sixty years of herding pigs would have led to her having noticed it. It’s possible that she fell in at night; or that she was carrying the pig and was distracted; or, as I said, she was thrown in as a sacrifice or punishment.
Perhaps she was in some unknown territory. A moment of inattention is all it takes. With all the pain from her arthritis and the cancer maybe she threw herself in to end it all.