skeletons are fighting if they see player , i will make it like steps :

1-spwan a lot of skeletons in one place (for they don't spread everywhere).
2-change game mode from creative to survival.
3-if you was far of them they will not fighting , but if you close to them... they will start fighting.
NOTE :even if you back to creative mode they will not stop fighting.
NOTE :in some times when remaining (four or five or three...) skeletons stop fighting.
(affect in 0.13.1 too)
samsung galaxy s4
4.4.2 (kitkat)