Joseph and the Shepherd

Another detail from the next paragraph of the Ethiopic nativity story The Road to Bethlehem is as follows:

Joseph looked around, and he saw a cave where animals were kept and he led Mary to it. Then he went off to find a midwife. As he went along the road, the earth trembled, and the oxen and sheep stood looking up into heaven.

Since this paragraph doesn't actually mention a shepherd, here's an icon that shows the scene to which this paragraph alludes (the animals are behind the shepherd on the right). Why does the earth tremble? Why do the animals stand looking into heaven? (Discussion in the post to follow in a day or so :)