Don't usually link to CB stories but this one has the ring of truth:

http://www.celebitchy.com/524394/star_taylor_swift_karlie_kloss_had_a_falling_out_over_tom_hiddleston/

Unlike the author I think it was precisely because KK had to hang out with Tayto on 4 July that she got annoyed with the whole thing. Imagine having to play nice for the cameras with your (girl?)friend's fake BF. That would really tick me off.