Unlawful arrest, lies and torture.
We all know the mantra. Trouble is little if any of it is true. He was arrested for flight and resisting. There were others in the wagon who did not suffer any discomfort. In fact they said he was deliberately trying to harm himself. This was a known drug dealer running from the police. No one witnessed police brutality.
He was not charged with anything prior to booking at the precinct, which never occurred.
In short nothing you repeated is verifiably true. All of the reasons for the riots are made up frauds. Once you get that much straightened out in your head you will begin to be able to understand what is happening.
No, the particular reason, the same reason for the other incidents, is resisting arrest.
Had Gray, Michael Brown, simply followed police instructions neither, include the NY guy choked too, would have happened.
Whether the arrest was justified or not could have been sorted out later.
These things do not justify, excuse, or cause rioting or looting and burning of private businesses.