Moray, the word today is 'escalation'. Just a few weeks ago, an officer tazed a person pulled over for speeding because the person was trying to point out a speed limit sign. A few hundred folks have died from tazings. Alternately, there's the old 'resisting arrest' or 'interfering with the operations of an officer' methodology that can be used to escalate the mildest situation into something deadly if the 'perp' isn't following the script.
This is why officers are held to such a high standard, and thankfully, the vast vast majority meet this standard. It's when the exception has a bad day that innocent people can die, and a seatbelt stop is as good a catalyst as any.