You asked me to explain why it happened and I gave you my opinion. That isn't the same as justifying it.
How do you explain it?
It was not an accident or the result of over-zealousness. It was deliberate and planned. To what end, is another topic entirely -- DON'T CONFUSE THE ISSUES, PLEASE.
I don't know what it WAS, but I sure as hell know it what it wasn't. It wasn't super-dumb-ass bikers acting out in the presence of God knows how many cameras, cell phones, and SIX marked police cars in broad daylight in a crowded public place. I sure as hell know it wasn't an honest mistake on the part of overzealous officers.