It could be Walk in Shadows wearing off causing Sneak to fail.
Were you behind one of the enemies, or behind all of them? On Explorer you would only have 3 skeletons to worry about, and they would all start facing the same direction.
Try bringing your mage into combat, so the summons have someone to target, and will be less likely to turn around than if they don't see anybody in range (if that is what is triggering the sneak to fail). I tried this (in explorer mode) with a single character, though, and after killing the archer, walking behind the melee skeletons and re-sneaking, they did not turn around after several times skipping turns. Attacking (and killing) one caused the character to unsneak, but after re-sneaking, the other continued facing away, so their sight cone didn't detect the character. Harder difficulties and stronger opponents can have better AI.
