To answer your direct question, there are two reasons. The first--which is normal and about which you can do nothing except stay in the room more
--is simply the fish's involuntary reaction to a perceived threat. My fish do this. I have a fish room with my 7 tanks, and only enter it specifically to observe the tanks or feed or work (water changes, etc) on them. The fish, being used to no one moving about in the room for most of the day, will naturally hide when I enter the room. This is a natural reaction to escape a predator. Once I am sitting in the chair for a few moments, life returns to normal. But if I make any sudden movement, they usually disappear (some are more prone to this than others). I have noticed that on the day I do the water changes, they are far less inclined to hide after I have been in the room for maybe an hour, moving around from tank to tank.
The second reason has already been touched on by Blackfeet. I realize the fish are still "small" relatively speaking, but they are now in much too small a space together. And this does cause problems that lead to stress and this weakens the fish's immune system making disease much more probable later down the road, and it may increase their natural aggression or just the opposite--make them withdraw to the point that they die from that. So the fright response may be heightened due to this too.
Fish grow continuously, and they need sufficient space throughout their lives to be able to do so normally. They also have behaviours that cannot be changed because the fish species evolved accordingly. Even if no physical aggression is observed, the pheromones and allomones released by all fish are chemical signals that other fish read, and this causes stress just as much. Stress is the direct cause of almost all fish disease and health problems; you can read more here: http://www.tropicalfishkeeping.com/f...um-fish-98852/