Fins were added to the boosters to make them stable. Many have done boosters on rockets and just let thme tumble after dropping off. The initial test of these booster droppers actually had one of the booster fall into the main rocket and damaged the fin. My design keep the boosters stable.
Also to aid in bolwing them off at the correct angle (booster nose cone outward away from the rocket), lead weight was added (shaded ared inside the body tube). It was quite a challange keeping the booster stable and having it still fall away with the nose cone tiing away from the main rocket body.