That was one of my favorite songs that we sang a capella in high school at our Christmas concerts.

In many parts of the world, it is the accepted practice for the audience to stand for this section of the performance. The tradition is said to have originated with the first London performance of Messiah, which was attended by King George II. As the first notes of the triumphant Hallelujah Chorus rang out, the king rose to his feet and remained standing until the end of the chorus. Royal protocol has always dictated that when the monarch stands, everyone in his (or her) presence is also required to stand. Thus, the entire audience and orchestra stood when the king stood during the performance, initiating a tradition that has lasted more than two centuries.

http://en.wikipedia.org/wiki/Messiah_(Handel)