I've definitely seen this behavior many times in all my mantids. I've just classified it as a mantis thing. lol I'm curious as to why they do it. It might be like a defense behavior. Like if they notice something is near, but not a real cause for display, a quick shiver can catch you off guard and possibly make you startled and back away. I guess it would only startle if their striking arms could make a sound. I don't know. A bit of a stretch I know, but it's a thought.
I'm just thinking about my mantis and I notice this during dusk or when it gets darker in the room. I put her on my pothos plant and if I walk by the plant while she's on it, she will flutter her wings and it honestly startles me because I'm not expecting it. So I just figured the rattling may ward off possible animals that are just in the way? I don't know. lol