Three rear-end collisions that reportedly took place after Cruise autonomous vehicles braked hard kicked off the probe, according to the National Highway Traffic Safety Administration.The agency also has multiple reports of Cruise robotaxis without human safety drivers becoming immobilized in San Francisco traffic, possibly stranding passengers and blocking lanes.There have been two reports of injuries related to the hard braking, including a bicyclist seriously hurt last March, according to the NHTSA crash database.Cruise spokesman Drew Pusateri said the company is cooperating in the probe, and that its vehicles have driven nearly 700,000 autonomous miles in a complex city with no life-threatening injuries or deaths.He said police didn't issue tickets in any of the crashes, and that in each case, the autonomous vehicle was responding to aggressive or erratic behavior of other road users."