Wicked paint reaction - why?

My brother shot his 71 Camaro yesterday and had some serious issues with a reaction between the paint and primer. What caused this? Luckily, it was isolated and most of the car looks great.

The primer used was 5 Star 5425 2K Urethane and the paint was H.O.K. Apple Red KandyBase. Clear was Valspar.

I used the same basic combination on the 70 Challenger I painted Friday and had zero issues