What's going on?
Ahh! OK so if you're introducing new birds, you're changing things and he probably doesn't like that. He could be reacting badly and unsure if things are safe. Hard to say, but the simplest answer is usually right with critters, and if that's your only environmental change, that'll do it.
My understanding is that the best technique for new introductions is to sneak them in after everyone else is asleep, then when they wake up they are assumed to just be part of the flock. The cage method will likely work too, I haven't tried it before.
Good luck with the new friend-making.