Why does light from the moon appear as a column when reflected from a body of water?

If the water were absolutely still it would act like a mirror and you would see the reflection of the moon as another image of the moon. But water is never still and since the surface is constantly changing angles relative to you then at some point every little patch of water is going to reflect the moon, even if its only momentarily. This only pertains to water that more or less is in line with you the viewer and the moon. Water off to the side would need more extreme angles to reflect moonlight back to your eye so the reflected moonlight tends to come off the water that lines up underneath the moon from your viewpoint.