Same thing happens to me when ordering a group of 2 or more slinger units to move to a new location -- some, but not all of the group will move...

I also get some weird unit movements when I order non-missile units to charge/attack -- the units will rush right up to the rear of a phalanx (for example) and then stop and sort of mill around and creep in toward the enemy unit. It's pretty apparent that someone in the unit called off my charge order!