Probably this:

http://onemileatatime.boardingarea.com/2015/06/14/united-str...

United gained a fleet of long-range 757s from the merger with Continental Airlines. These are smaller planes (but ETOPS certified) with smaller crews, which is good for United's bottom line.

757s can make it across the Atlantic, as long as conditions are good and there are easy winds.

On the westbound return flights, and this happens more than United will care to admit, the headwinds can be too strong and the flight needs to refuel in Goose Bay before continuing to the United States.

This typically adds an unexpected 90 minutes to the flight, which can really screw you up if you're trying to clear immigration and make your connecting flight. You usually don't learn of this fate until you arrive at the terminal for boarding.

Goose Bay in Labrador and Gander in Newfoundland are the old refueling stations between the US and Europe (originally Shannon Ireland). They're still capable of handling large jets so they're typically the landing spots if there are fuel or other issues such as medical emergencies with planes flying over the North Atlantic.