My DH does this all the time, every converation is accompanied with mad gestures, waving hands and manic arm movements. Drives me mad, I just stare pointedly at his arms and hands while he is speaking and it stops it for about 10 seconds, and then it starts again!
I agree with 25Avalon, it's a nervous habit that the sufferer is generally unaware of until it is pointed out. One of my DDs had the same habit until she saw a replay of herself in an interview situation and realised what she was doing. She is now much more aware of it and uses hand gesures much more economically and to more effect.