Wiring up ATX PSU to power (several) 12V LED strips
72W at 50% PWM on 12V is 36W at some low frequency like 1kHz of AC ripple current which is excessive for most decoupling caps in ATX PSU’s.  

If output caps have a dissipation factor which determines what % of load ripple power must be absorbed by the cap.  Aging increases this rapidly.

Although a typical MOBO has dynamic CPU loads the change rate in frequency of load cycles is much lower.

Conclusion;

Excessive ripple current for YOUR PSU.  Overheated cap, slowly turned into short circuit and faulty circuit protection.  Get a better LED driver supply.