Exactly the point that needs to be made here. Why would the US send ships to evacuate Americans from Israel? Only if there is going to be an attack on Israel. An invasion by Arab armies does not seem imminent so what could it be then? Missiles from Hezbollah and Iran in addition to the ones from Hamas? Is there something that the White House is hoping for that Israel should be preparing against?
The odd thing is IIRC, Obama and/or Hillary took their time with evacuating people from one of the Arab Spring countries. I remember at the time they waited longer than seemed prudent- I think it was Egypt. Now, they do this? Either they know something is coming very soon, or there is a political reason. Probably good time to fill up gas tank.