KarmaYoga

Perippa, this is what I feel could be an explanation. We are born as a human in this birth due to our Karma in our previous birth. How we utilize this birth would be a deciding factor for our next birth.
If we take the example of the story you cited, the sanyasi was obviously still attached to his body. He was definitely not a completely liberated soul. Probably the fact that he would be born as a pig in his next birth, was a test that was put to him. If he had not reacted so adversely to this news, then even that could have been nullified.