Well if tether went to say 90c and dropping, tether holders would certainly try to swap them for bitcoins but I'm not sure the bitcoin holders would be keen resulting in bitcoin going up in tether terms but probably down in US$ terms as tether fell.
BTC holders would not look to exchange for Tether in the case Tether turned up unbacked. If BTC sellers were to sell they would exchange for other cryptocurrencies or more likely other stablecoins that are actually auditted.