To put it in context:
http://www.cowboylyrics.com/lyrics/mcmurtry-james/memorial-day-20599.htmlhttp://www.youtube.com/watch?v=aLjj7FD-CAoI think that song is about family, about how families get together even though they have arguments that last for years - that they drive for hours to to be together and then spend half the time screaming at each other over things that happened years ago. The narrator doesn't really understand it but two of the older kids (his siblings?) assure him that this is just what happens when they all get together.
Mary doesn't cry during her switching, she just looks at her dad with betrayal in her eyes - accusing him of not intervening to stop the switching (the narrator doesn't quite get that so wonders why Mary is so mad). This becomes a new wound, and on the long drive home no one talks about what happened. Mary, and probably her mother, are too angry with dad's non-action to speak.
Thanks for pointing out the song - I'd encountered the singer before but hadn't stumbled over this one. Another song (We Can't Make It Here) from the same album was used during the closing credits of a Sopranos episode - and is a bitter account of the state of the nation. James McMurtry is good at writing observational songs.
Goodgulf