I'm not sure we're talking about the same thing here.
I'm not talking about a police officer seeing a cracked taillight and concluding that it's an indication of a possible crime.
I'm talking about a police officer stopping a car for a cracked taillight, and during the stop, the driver's demeanor, behavior and responses trigger the police officer's sense that something is amiss. If the driver refuses to provide a driver's license, registration or insurance, or claims he doesn't have it with him, if he has an opened gin bottle rolling around on the floor of the car, if the driver's answers to questions are non-responsive to the actual questions that were asked.... and so on and so forth. A person's behavior gives a police officer a lot of information about that person.
When red flags get set off, when the officer's "hinky meter" redlines, when the driver isn't behaving the way normal people behave during a minor traffic stop, that's when the officer might decide to ask the person to step out of the car, perhaps pat them down for any weapons, look through the windows to see if he can spot anything openly visible that shouldn't be there, ask for permission to search the car, and in general see what he can find out about what's going on that person and why they're behaving so weirdly.
Traffic cops have many, many interactions with people every day. Most of the time, it's an otherwise law-abiding citizen who has malfunctioning equipment, who was speeding, who rolled through a stop sign, etc. It's a normal, civilized interaction. The driver gets their ticket or their warning and off they go. The driver isn't happy about the ticket; no one is. But they still behave like normal, civilized people. Then there are the other stops, when the person behaves ... oh, perhaps the way SD behaved. Those are the stops that set off the hinky meter and caused the police officer to dig a little deeper. The officer didn't shoot him because the car was missing a front license plate, nor did he shoot him because he had air freshener in a gin bottle. SD's own behavior cause the police officer to ask him to step out of the car. And SD's decision to drive away rather than do as requested led directly to the shooting.