Its not like every chip will fry at 15v...
Some cars are heavier and some have more mag force than others which adds strain to the motor... hence pulling more amps...
some motors are just naughty and pull more amps than they should through wear or bad manufacture...
The more cars you run at a time and the more XLC's etc etc that are in the track the more of the Amps will be sucked away.... placing less strain on the chips...
Running no mag will reduce risk to chips at 15v... its the "borderline" figure for running with magnets...
Small circuit, solo running, high voltages with magnets..... could be a "perfect storm".... Assuming thats a Carrera GTR-500 (lovely car) it says in the blurb "Car partly equipped with patented Variable Magnet System (VMS) Supplied with magnet units included" Not seen this mag setup in the flesh but could just be that its compounded the problem... Plus new cars have "tight" bearings and "noisy" motors straight from the box after a few minutes of running in and a bit of grease these issues reduce.... you were probably a bit unlucky... 13.8v proven always safe.... ENSCA runs that as a fixed voltage... Never had a problem.... Was recommended by RichG to for the club to run at that level when he did the PB for us.