That's an area where you have cold water mixing with the edge of the Gulf Stream. It's June, warmer waters moving up from the Carolinas cool, and the continental shelf juts out from St. Johns, Newfoundland which - could - influence water movement at certain depths intermittently. This phenomenon happens in other places, Hawaii comes to mind, but I think we run into them here on the Central California coastline once in a while.
Keep in mind, we don't send people this deep very often, and I don't think down currents are a big issue with ROVs. Think down-draft with airplanes. Any time you have different temperate masses circulating, cold wants to go down, warm wants to go up. At the ocean's surface this can cause rouge waves, we discovered this off South Africa. The issue is Marine Science doesn't go deep as often as it can in shallower waters due cost in equipment, fuel, access to vessels, etc compared to studying coral reefs, or intertidal regions that are close to shore. There's still so much to learn about how the water column works in the open ocean, but my money would be on a down-current. That or the ballast on Titan was not properly set. I know that even on a quality submersible that sometimes the ability to surface is not an automatic even in good conditions. I'll wait until an official report is completed, I don't like speculating to the extent I already have with this case.
