I had dota opened up on Marci while eating dinner (coincidentally) and saw her do this animation. It seems to be super rare though because I had to wait almost 10 minutes for her to do it again to get just these two screenshots.
It seems to be unfinished, like a few other of her animations. I don't know why they would put this in the game already, but here it is lol.
She doesn't chew, and her face doesn't change at all while the animation happens, and it overall is very strange. But hey now we know why there is a drumstick with her model!