Just my opinion but it could be many things. First from my observations most fish stay away from human interactions. My fish tend to stay to the side that gets less people walking close to the tank or the side furthest from the TV, or noises, things like that. Second it could be flow. My fish will swim through the flow but tend to hang in areas where the flow is gentler and more dispersed. They will swim the whole tank but rest in these areas. Third could be the scape. They will hang where they feel safest (Which goes along with the first one in my tank). In my tank the side of my couch sits on the left hand side of the tank, and my TV is towards the right side so mine tend to stay to the left as well. When I get up from the couch its towards the right so the fish gravitate left to feel safe. All but 2 sleep in caves on the left side so that is sort of their homes. the other 2 are the newest so they sleep and tend to stay to the right to avoid territorial issues. Another thing is coral placement, fish will have different depths in the tank. Your torch coral on the right is down low and they can sting fish so that might keep them low and to the left, where as you free swimming fish will stay right and up top because of the torch on the left (which is higher). Feeding is another thing. I tend to feed on the left side so they might feel more comfortable there because they are waiting for a meal. Just some thoughts and observations from my tank, But I could not tell you for sure what goes through their beautiful little minds. try feeding then on the right or changing the flow if you want to get them coerced to the other side.
