A six-car pileup in Park Slope that sent five people to the hospital on Thursday night was allegedly caused by an illegal drag race, according to first responders and multiple witnesses.  “They were drag racing,â€� said a responding firefighter, who declined to provide his name.  Authorities arrived on the scene near Fifth Avenue just before midnight to find several damaged cars scattered in the street and on the sidewalk between Fifth and Sixth streets.  Paramedics took five people to nearby Methodist Hospital for treatment of their injuries, according to a Police Department spokesman. Authorities inspect the damage. Witnesses at the location recounted the deafening sound from the accident.  “There was a super loud noise …