My scarlet does this all the time, and has for years... idk why he does it.. he's very aggressive though... so when he's in that position, I leave him be lol.
Sometimes the biting can also come across as begging.
My Macaw will lower his body and shake is wings at first to beg for whatever I am eating of come out of his cage, If ignored, he will place his foot into his mouth too but never bites down on it. It appears judging by the video that he wants the bottle and when it is dropped, his bite appears to tell you get the bottle and give it to me. Joe