It must have all gotten very serious...
Either that or, as was noted before, he's just totally mimicking Iron Bull's body language. Whether he'd do that to compete with him or mock him, though, I'm not half as sure. Speaking of which:
Dorian: *mocking tone* Ooh, look at the big, scary qunari, standing there like a bloody statue! I'm so intimidated by your ridiculously small head and your crossed sausage-arms!
Iron Bull: *small smirk* At least I can keep still for more than a couple seconds at a time, Magister.
Dorian: *frowns* I can be still! Being still is simple, it's child's play, it's... it's just not as entertaining, that's all!
Iron Bull: *smug* Sure, you can stay still. Oh, and did I mention that I've been the Empress in disguise this whole time?
Dorian: *flatly* Hilarious. *points at Bull in a dramatic, accusing manner* You don't think I can do it, do you?!
Iron Bull: *looks pointedly at Dorian's flailing hands* Uh, no. No I don't.
Dorian: *huffs indignantly and throws his hands up in the air* Fine! I wager two sovereigns that I can keep my arms folded throughout this entire meeting!
Iron Bull: *raises eyebrows, considering* ... I'll take that wager. We start when the Inquisitor walks through the door.
And throughout the entire meeting Varric and Sera are exchanging concerned looks about the two's identical body language, and the Inquisitor thinks Dorian is ill or something because he isn't gesturing emphatically whenever he makes a point, and has to visibly strain himself not to do so.