Cooper's hawks prey almost exclusively on small birds. It looks to me like your bird started an aggressive run at something that was behind the post (or whatever it is that's blocking the camera), then slowed and swerved while near the ground. Maybe there was a small bird perched on or under that low bush, but the hawk decided it'd missed its chance. Maybe the hawk was making a blind run based on noise (they do that, it lets them catch their prey by surprise without being seen), or maybe it even was confused by a falling leaf or a reflection. In any event, it decided to rest on the ground for a few seconds to catch its breath (and lighten its load) before promptly returning to a safer perch with better views.
Note that when a hawk of this type strikes a bird mid-air, it generally doesn't hang on to the prey. The stunned prey drops to the ground and the hawk picks it up from there. So going to the ground briefly is pretty normal.