Your hawk probably mistook the black chickens in your flock for crows. Hawk predation on crows is the reason that crows hate all raptors and with sufficient numbers of crows on hand will attack at the drop of a hat.
Crows are a handy food source for both hawks and falcons. Its just when crows gang up to persecute hawks that crows are a deterrence. Hint!!!!!!!!!!!!
Keep your ears pealed for crow chatter. If you hear a murder of crows going ballistic I recommend that you check on your flock immediately...