I have heard (very much unconfirmed report) that she was being arrested for a Harris County warrant, which is why she was out of the car. Don't know how it spiraled south from that point, but a witness (the person who shot the video) claims on FB that she got very belligerent, and did indeed assault the officer on the scene.
From everything I have read to date, though, the arrest itself does not appear to be related to the death in the jail 3 days later. The family held a press conference a few hours ago stating that everything they know about Sandy leads them to believe that there is more to her death. But they also concede that they are open to believing that Sandy did, indeed, commit suicide if presented with facts that corroborate that story. Most of all, the family asked for patience to let the investigation happen and not jump to premature conclusions.