We had just pulled away from lights he drove normally then simply hit the brakes hard
I had a similar accident many years ago, pulling away from a junction with just the one car in front of me and nothing else in sight in either direction. He slammed the brakes on without any warning and I went into the back of him. But it was totally innocent - turned out to be an older guy in a brand new Volvo ... an automatic, which he wasn't used to driving. He'd taken his foot off the brake so the car moved off, then plonked his foot down on the accelerator but hit the brake pedal (which of course is much wider than on a manual car) again by mistake. He immediately said what had happened and apologised profusely, but at the end of the day I was still too close to stop in time - lesson learned. Getting a bit too close is pretty common at junctions and roundabouts, which I guess is why scumbags often choose these locations to stage accidents nowadays.