Speed up then lock up!

Riding along North Lake Road on our Saturday group ride, a driver who had his mobile phone in hand, waits for the last cyclist to pass him before he proceeds moving again. He seems to be accelerating quite fast behind, which was kind of worrying when you think what could have been! He then brakes hard directly behind. Was this due to a lack of attention and not looking forward until the last moment, while still having his mobile phone in hand?