When you walk into the room where his tank is located, are you approaching him from behind? If so, he could be seeing your shadow falling on him and in front of him, and that may be freaking him out.
In the wild, many predators that see beardies as food will approach them from above and behind, such as birds, or other larger animals, so its become part of their survival instinct to be wary of anything that approaches them that way. If that is what's happening, then try to position his tank in such a way that he can see you coming, rather than coming up behind him. Or you could also try saying his name, or speaking to him as you enter the room, so he knows that its you.
I know all three of mine, on the rare occasion when I harness them and take them out in the yard when the weather's warm, will completly freak out and go into a full display if a bird flies anywhere near them. Also, we live very near the glide path for Mercer Count Airport, and you wouldn't believe the reaction I see whenever a plane or helicopter flies overhead. They're MY BABIES, and they can make themselves look so ferocious, that I won't even consider picking them up without putting a towel over them first. Of course, we all know what little sweeties they actually are, but they would probably scare someone who didn't half to death........a beardie in full display is a very impressive sight. Its easy to see how they can use that ability as a defense mechanism to scare away animals who may try to harm them.
If you are approaching him from behind, or even from the side where he can't immediately see you, the I'm pretty sure that that's probably what's happening. If it isn't, then I'm afraid I havn't got a clue about might be frightening him, unless you've only had him for a short time, and he just hasn't learned to trust you yet!