The situation isn't too complex: Bitfinex is a crypto exchange that had difficulty finding banks that would work with them. Banks are regulated and expose themselves to a great deal of liability if they provide banking for services for illegal endeavors. As you can imagine, some crypto exchanges are deemed too risky.

Tether and Bitfinex are the same thing, though both will vigorously deny that, but it's patently clear that they're intimately related. Tether has variously been able to get banking services, and thus act as a USD deposit to enter crypto exchanges like Bitfinex (USD -> USDT tether -> trade for Bitcoin, etc.). Because it's so clear that Tether is simply used as a way to circumvent AML/KYC, Tether continues to have difficulty securing banking services.

This current uncoupling of USD:USDT(tether) is due to such a banking issue. There are two major risks with Tether:

* Liquidity: without access to banking services, Tether uncouples and there can be a 'bank run' as the majority of funds are unavailable to maintain the stable price.

* Reserves: right now there are no guarantees that each Tether is backed by actual USD. You simply have to trust Tether on this. Ultimately Tether may be a sort of fractional reserve, and thus enough selling pressure will make Tether worthless.

At it's core, it's just a clumsy obfuscation.