If your secondary spins while running could be caused by a couple issues:
1. Clutch misalignment from the factory. The belt is touching or very close to one of the primary sheaves at idle, the weak factory clutch calibration masks this problem sometimes as the belt tension is low but when you install a proper clutch kit such as KWI kits, this factory misalignment now shows itself. Your options are to either have BRP warranty the clutches for a new set or send your clutches in to KWI for the FULL FLOAT MOD, then you can adjust it as required and this will never be an issue.
2. Damaged primary clutch bearing. If the bearing is not spinning smoothly or seized on the shaft then it will drag the belt and spin the secondary.
3. If you have KWI Full Floating clutches and this starts happening, most often it is because the clutch is not sliding on the gearbox input shaft smoothly. remove the secondary clutch , clean and re grease the shaft and inside of the secondary clutch shaft bore.