Pretty sure it was just because the word 'yank' was accompanied by the words 'S***house', 'gutless', and 'cowards'. As if 'yank' was supposed to be derogatory as well.
I suppose its all about context.
But ive never considered the word offensive, like the word limey, I mean thats meant to be offensive, but if I was a called a limey F, I doubt I would take offence.