* Traditionally, classical musical concerts end with the audience standing. People may stand for the finale to show appreciation for the performance they've just seen, and some may regard the Hallelujah Chorus as serving a similar function in a religious setting.
* The Hallelujah Chorus is a particularly jubilant and uplifting piece of music. Some religious music enthusiasts may feel prompted to express their joy and gratitude by physically getting to their feet.
* Finally, some people may simply stand for the Hallelujah Chorus out of respect for tradition or social convention.