I think i've noticed myself that higher vitamin C doses seem to stimulate bile production. My† looser stools seemed to have more bile in them(distinctive yellow color). My theory would be that the acid is somewhat irritating the Gi tract causing a stimulation of bile to be released. This might be the body's way of trying to push the substance out of the body faster due to PH disturbance(too acidic for the gi tract).† Bile stimulates peristalsis.
I've read that good bacteria like a slightly acidic environment. But an extreme acidic environment might not be suitable for them. Excessive Vitamin C flushing might disturb the proper Ph in the Gut.