Seeing that would have been very interesting. Maybe you did what that squirrel did to those blue jays.
It's fairly common, and fairly predictable. They nest in tall grass, in marshes and meadows. The males, and possibly the females as well (after the eggs hatch) will perch in small trees, cattails, or similar spots overlooking the nest. Get within a critical radius of about 20 feet, and they'll let you know they aren't too happy about it. The 'attack' consists of hovering 5-10 ft over your head, screeching, and occasionally diving to within 2 feet or so.