Velleman PKA'03 Motor Drivers burnt

I am using 2 Velleman motor driver shield to run 3 24V DC motors. My circuit is well tested and no loose connections. However, I was trying to tune my PID controller and the motor driver blew up. I would appreciate if someone could shed some light on this matter i.e. what could be the reason?

Is it due to current or Back EMF, or too high frequency ?