OK I've figured it.
As the Marverni I thought it would be a good idea to have their Horn Blowers attached to every unit to keep up morale.
However when these guys are attached to the slingers they cause the entire unit to forget about its slings and charge in with the daggers. Once a Horn Blower is removed from a slinger unit it will revert to using its sling again.
Whether this is a bug or W.A.D. I couldn't say. Maybe one of you guys in the know can add it to the bug thread if it is needed.