Currently these are just random.

But instead of that.

You should be able to counter attack enemies by killing them right before they attack.

This would cause them to drop an attack up orb for everyone.

Or get a bonus for killing enemies from behind. This would force them to drop a movement speed booster.

Defense bonus orbs would drop from enemies that are inflicted with 2 statuses at the same time and then killed.

This "style bonus" would serve to make the game more skill based, even on lower levels. When a kill is made in the appropriate fashion to cause a drop, a sound effect would be heard.