Bitfinex made the decision to cut off US clients a few months ago, so I'm not surprised with that, but the Bittrex situation is quite bizarre.
Maybe the U.S. regulators, even the Trump admin, may be putting pressure on Bittrex to cut off certain users.
Remember how the Justice Department put pressure on PayPal, Visa and MasterCard to stop services to Wikileaks? It would not surprise me one bit if we see similar maneuvers regarding the crypto exchanges.
In August, Bitfinex basically told US Customers to "start packing your things," and yesterday they gave them their deadline to do so. This is why I believe, even though this situation isn't great, that Bitfinex handled things professionally.
However, Bittrex is a different story, because it seems they're randomly closing accounts without responding to requests and telling people the reason. Get your coins out of exchanges!