ThinkProgress writes about What The Supreme Court Has To Say About Sandra Bland's Arrest.
Rodriguez v. United States held that police could not extend the length of a routine traffic stop, even for just a few minutes, absent a safety related concern or reasonable suspicion to believe that the driver may have committed an additional crime. As Justice Ruth Bader Ginsburg explained in the opinion of the Court, ‘[t]he tolerable duration of police inquiries in the traffic-stop context is determined by the seizure’s ‘mission’ — to address the traffic violation that warranted the stop, and attend to related safety concerns.’ A police stop ‘may ‘last no longer than is necessary to effectuate th[at] purpose.’ Authority for the seizure thus ends when tasks tied to the traffic infraction are — or reasonably should have been — completed.’
By the time Encinia asks Bland to put out her cigarette, the ‘mission’ of his encounter with Bland is almost at completion. He has already written the citation and brought it to Bland. While she is being handcuffed, Bland even indicates that she was ‘trying to sign the fucking ticket’ before Encinia tried to pull her out of her car. Had the officer not decided to extend the length of the stop over the argument about the cigarette, it is likely that Bland would have been sent on her way very shortly after she declined to extinguish her cigarette."
This is just a crazy story and there are so many parts to it. She evidently did change lanes without signaling so the stop itself was ok. Enchain clearly escalated it because of her belligerence but that's entirely on him. The above says the escalation was illegal. I note that in the video he orders her out of the car but doesn't say she's under arrest until she asks, which seems a little backwards to me and in spite of her asking repeatedly about the charges he never answers, which I don't think he's allowed to do. So she pissed him off, which is legal, and he went too far, which isn't. There's probably excessive force involved too, but that's off camera and not clear.
Now that's just the arrest, how she ended up in prison for several days and then dead is another (very suspicious) matter. If she couldn't raise bail, the time is plausible (though ridiculous).