Did you issue a group command to your slingers? Units in group tend to respond weirdly. Even worse when you issue command to group A, some units in group B might overheard your conversation and decided to move accordingly.

It is actually good to have a lot of missle units on a field battle. For archers and slingers you can put them into a single line but deeper formation depth. This way there won't be any friendly fire problem. In STW (don't know about MTW, didn't really play it) the same unit also head-shot the guy in front - that I think is even more stupid.