It's highly possible that the trigger is intimidating the lion into hiding. People tend to see things such as this in black and white. They think you've either got a peaceful reef community tank or a predator tank, without considering that certain predator type fish won't mingle well and should have their own separate tanks. Novices tend to assume that lionfish, due to their name, predatory reputation, venomous spines, and shear size, are vicious beasts that won't hesitate to tear apart any tankmate that get's in their way. However the reality is that lionfish are very docile, slow moving fish that won't do any harm as long as their tank mates can't fit inside of their mouths. Lions normally swallow their prey whole, whereas a fish such as a trigger is capable of ripping a fish apart and eating it bit by bit, making them a much more unpredictable predator (I once saw an Indian trigger rip a sailfin tang in half that was of the same size). A lion, with their slow swimming and flamboyant finnage would make a perfect candidate for a trigger to harass and nip at at, especially if the lion is smaller.