I have never figured out why they hide. Here is what I have read:
They will hide if they are getting too much light. After a while they are supposed to come out again.
They will hide if they are going to split. Not sure about this one.
They start moving around whenever there is a change. Usually this means a lighting change or change in water flow patterns.
Best way I have found to control this is to place them on a flat rock that is not close to a wall or other rock. They tend not to like crawling along sand or floating.
however, I have seen them do both. I have had BTAs float through the water (scary), and seen then crawl over sand, so nothing is certain.
Good luck, Kevin