The game was temporarily stopped after problems erupted soon after Cunha had doubled the visitors’ advantage with 12 minutes remaining.
West Brom’s Kyle Bartley went into the crowd to take out his children amid the ugly scenes, which occurred in the corner of the West Stand and Birmingham Road End.
It was part of the ground which theoretically should have been holding solely home supporters but there was a lot of pushing and shoving, with some fans spilling onto the pitch.
Police and security rushed over to the area to quell the problems but it was a long time before order could be restored and both teams were taken back to the dressing rooms before play later resumed.
The Football Association confirmed it has launched a swift investigation while West Midlands Police said two people were arrested after the disturbance and another prior to the match for possession of an offensive weapon. A man has been taken to hospital with head injuries.
