I remember reading about that in a book called "Astonishing-And True-Stories of the Speedway" by Al Powell in a chapter called "The Missing Pole". Apparently when Palmer had been practicing he was using the flagpole as his marker of when to start braking. What he didn't know was that the flagpole is removed on race day, by the time he realized it was gone, he panicked and locked up the brakes.
After this incident, only drivers who have actual and sufficient racing experience are allowed to drive the Pace Car.
The author of the book, Al Powell, was a race car driver who was stricken with polio in the 1950s, which left him a quadraplegic, but apparently did not slow him down.