We don't know what negotiations and plans are going on in the background for evacuations. For example, the UK only just in the last few minutes announced that it would be doing evacuation flights but not the details. For all we know, it may be the case that the US government is setting up plans to evacuate US citizens to ATH (either by plane or ship which is what they did with the Lebanon war) and has given UA the heads up to start extra sections there. It's easy to armchair quarterback when you aren't directly involved in the rapidly changing situation.