The woman then came back and started punching, pushing and yelling abuse at the female driver, who had got out of the car.
She said members of the public tried to calm down the woman, who was "yelling and screaming" at the driver.
She was yelling, there was a pregnant woman in the vehicle, the witness said.
"She just flipped."
The witness said the driver was "shaken up" and was looked after by members of the public until police arrived.
A nearby worker who also witnessed the aftermath of the crash said the female driver was punched in the head and upper body.
"She was pounding the top half of her body."
The alleged attacker left in an ambulance along with a pregnant woman, the worker said.
Inspector Barry Smalley of police northern communications confirmed two women were involved in a "physical argument".
No arrests had been made last night.