How to reproduce:

1. spawn some bow-wielding skeletons with skeleton spawn egg
2. Connect this command to a constantly-running redstone clock:

/entitydata @e[type=Skeleton] {NoAI:false}

3. You'll see that skeletons close in to you as if they wanted to hit you with a sword (but they don't harm you in any way), instead of keeping distance and shooting you with a bow. The same thing applies for wither skeletons.