So, for a quick solution, for the ones you do not want to move, set phys to enabled but make them static. That would be a work around.
To confirm, they are meshes, not brushes?
If you change the projectile back to a normal projectile, does it still happen?
If not, I would add the particle effects bit by bit until you find where it is stuffing up.
Is the second particle effective spawned upon hit?