I'm only guessing, but it may be the crypt fiend's projectile, which goes to the target, and then returns. As far as I know the CF is unique in having this "boomerang" projectile. Try giving it to a "standard" ranged unit (a priest, for example) and see if the priest then uses "boomerang" projectiles.
it does return but only the spider attack not the animation u added as the projectile so unless u use the crypt fiends attack animation it looks stupid