With so many crows in town it's inevitable they'll encounter a predator they don't like. Pity the immature Coopers hawk in this picture!
As a species Coopers hawks have enough moxy to cope with crows but 3 to 1 is stretching the odds.
I'm sure the crows started it. A "Coop" is not going to eat a healthy adult crow but the crows remember what the hawk can do to their weak youngsters, so when they found an immature Coopers hawk they decided to test their strength, maybe have a little fun at the hawk's expense.
They're serious about this fight but it's not life threatening. Eventually it breaks up, no one gets hurt, and everyone involved learns a valuable lesson.
The crows learn about cooperation. The Coopers hawk learns to avoid gangs.
(photo by Steve Gosser, September 2008)