I'm no expert but mulling it over.... If Tether is big enough that the price of BTC is effected by it, such that investors feel they can cash out and that is part of their risk calculation, then a tank in Tether would also cause a drop in BTC.

If Tether is backed by loans and not hard cold cash that's a risk and effects the value.

If someone is able to buy Tether on credit, and the credit is actually backed by BTC then there could be a loop.

I'm not sure about the likelihood of this but if Bitfinex is overleveraged somehow then anything is possible. At least I think that was the argument.