She's probably just wary and been assaulted on the tube a lot. There are a lot of pervs on underground trains, for some reason, especially public masturbators. I suppose it's because they have a captive audience. I seriously avoid taking trains in the summer, because for some reason, summer makes them braver or hornier.
And it's not just once that it's happened to me that a guy sits next to you, you're reading or listening to music or whatever, and then they slide next to you a little more, and then they lean on you a little more, and you continue ignoring them, until they finally start to touch you and by then you have to get up and leave because no one else pays any attention or tells them to stop.
(18yo girls probably catch the worst of it, too.)