There are a lot of possible explanations for such an action IF that is actually what happened: from error, to panic, to anger, to a determination to shut him up.
One of his officers put his cruiser into gear and jumped out of it, letting it roll at the suspects to draw fire, he said. The suspects peppered the car with bullets.
After several minutes, the elder brother, Tamerlan, walked toward the officers, firing his gun until he appeared to run out of bullets, Deveau said. Officers tackled him and were trying to get handcuffs on him, when the stolen SUV came roaring at them, the younger brother at the wheel. The officers scattered and the SUV plowed over Tamerlan Tsarnaev, who was dragged briefly under the car, he said.
Dzhokar Tsarnaev abandoned the SUV almost immediately on a nearby street and fled on foot, triggering an all-day manhunt.
Let the speculation begin.