I hit the car in front as it braked very suddenly. Why am I at fault?

I was driving down the road when I reached a junction, there's car in front of me queuing to turn right and the driver accelerated so I thought the driver is going to turn its way so I follow his back. Out of sudden the driver stopped his car as on the opposite lane, a bus was coming very close to him. Therefore I braked immediately however it was too late so I banged to his right side rear bumper and the bumper had gone in an inch.

We stopped at a place to avoid traffic and straight I told him let's go workshop to deal with this. So we reached a knocking workshop, the guy had a look and checked on the bumper straight away quoted RM300 just to fix the bumper without paint.

In this kind of situation, I would like to know was it my fault?