The YouTube description provides the following context:
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It all started when a cop stopped a man for jaywalking. Things escalated quickly from there as voices were raised and a crowd gathered. When two young women intervened an onlooker pulled out his camera phone and started to roll tape.
The altercation took place yesterday on Martin Luther King Junior Way South. The tape begins as the officer appears to be trying to arrest one of the girls for jaywalking.
Both girls, ages 19 and 17, were eventually cuffed and booked on suspicion of obstructing an officer. The one who was punched was checked out by medics and deemed to be OK.
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Thanks to Jefferson for the heads up.