In the very beginning of the movie, when they are buring Prince Imotep in the ground after they had mumified him, they are placing the sand on top of his sarcofagus. Therefore, he is being buried like normal. However, in the scene where Jonathan is "playing golf" and he knocks the tomb down, and it shows where it came from, the sarcofagus is not placed the same. It shows that it was standing up in the ground instead of lying flat. It would be impossible for it to come out that way as shown from the way it was put into the sand to be buried in the first place.