Strange Fireworks Over Noe

A reader wrote to tell us about this odd sighting tonight:

We went to the top of 21st and Sanchez tonight to catch the fireworks over the Bay. About 150 people or so were there with their friends, family and dogs at 9:30 enjoying the view and lights. At 9:42 as the main fireworks were really getting going, a cop car rolled up – and pushed its way through the crowd. The crowd booed the single policeman as he forced his car through the seated crowd, making every get up if they didn’t want to get flattened. As the cop was rolling away, a homemade IED firework went off in the middle of the intersection of 21st at Sanchez. The paint-can-sized firecracker/sparkler went off for the next 2 minutes, kicking off smoke, noise and flares into the crowd. A few people clapped nervously but it was weird and a little scary. A few minutes later the fireworks over the bay ended and the smoke and fire spewing paint can was gone. A little boy walking by afterwards asked “Why did the police do that?”

Why indeed. Anyone have any details about what really happened?