I'm sure it is flow and lighting related. Like a plant that needs more light will grow as tall as it can, a coral will probably grow to ease the strain of flow and find brighter spots.
That being said, predicting such growth is probably very difficult with coral. But I don't really anticipate you having a problem. That Hammer looks fairly mature, so it is *probably* as tall as it will get. Always possible it could grow taller, but assuming all of it's needs are met, I don't think it will.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef